The rules on teens dating and age ranges varies by state.  You should probably look into what your state considers legal or illegal.

In some places it's not going to be an issue if they are fairly close in age.. like in TX.. within 3 years as long as younger party is over 14.   so a 15 and 18 is ok.. 16 and 19 etc.. 

So, before you sound that alarm... I would try to know the facts.

As far as advice and counsel for B/M 15stb16.  I am assuming this guy is in the final semester of his senior year... is it really a good idea to get into a relationship with someone who will be very soon moving on to a new stage in his life.  It might hurt her feelings but as this guy expands his world beyond HS.. on to college or the military etc.. he will be running in circles with people who are much older than she is and the desire for him to want to have a "kid" for a girlfriend is likely to disolve as his peer group and potential dating targets are all out of school and leading more adult lives.  It may sound unfair.. but a guy out of HS is just in a different world and place in life.. maybe in a few years if they are both single and out of school they might reconnect at some point.. but right now, the timing is destined to cause heartbreak.  He is going out into the big old world.. she is still in her childhood stage at home going to school.

As far as her wanting to not tell her dad.. or not wanting you to spill the beans.  Here is what I say about that.

If you want to make "big girl" decisions on dating someone.. you have to make the big girl decision to be open and honest with your parents.  If you feel like you have to hide something.. that is a clear sign that it probably isn't the right thing for you to be doing.  If, despite the warnings of life changes, don't dissuade her from wanting to date.. then all the parents need to meet the kids.. understand the potential risks if things go wrong and she does get pregnant etc..   your DH needs to meet this guy.. meet his parents.. understand if they are the kind of people he even wants his daughter to be around... and make sure they know who he is as well.

So.. she can tell your DH herself but you won't be hiding this from him.

For what it's worth Spawn at 15 secretly began dating a 20 year old, of course fully supported by Meth Mouth.  There really wasn't much we could do aside from not allow her to see or speak to him while she was living with us.  That didn't deter her or the D-Bag guy.  DH did speak to the parents of D-Bag guy and let them know he was not on board with their adult son dating his minor daughter and if she ended up knocked up he would press charges.  The D-Bags parents are "important" people in our community so nothing was going to be done by the police.  Spawn dated him the rest of high school and for a bit after.  She started cheating on him with another older guy and so her story has continued.  I know she's currently dating a older man with daughters...I hope she remembers that when they become teens and want to date older guys. 

Yup, tell dh.

He should know what YOU  know about his daughter.

Dh already approved his 15year old going to the prom with 18yr old dude. So, he is not a new random guy. He is still 18 though. 

Still, dh should know what YOU know about his daughter.  

DH can take it from there. 

JMO

 "we dont plan on doing anything  Said every teen ever even if they have plans

 and its not going to be like that and hes very respectful  Until they are alone together and hormones override brain

and wants permission from both my parents If they want permission from both of her parents, than SD needs to tell her father.

and his parents dont care they just think we should wait until Im 16".  His parents are trying to hold him back knowing, as ESMOND said, that this relationship likely has a short shelf life. They are hoping it will be over before she turns 16.

I think its normal for an almost 16yo to be dating an 18yo.  I was dating an even older guy at that point altho my parents weren't thrilled.  So when I broke up with him and started dating exDH who was 2 years older, they were probably relieved.  Little did they know.   Lol.

So, yes, if BF wants to make sure the parents are on board, that solves the "don't tell dad" issue. She can ask him to dinner at your house and that will give DH ample time to check him out.

The bigger issue is birth control.  She's going to be seeing him regardless what you, DH, BM, his parents or anybody else thinks.  We all encourage you to disengage so I'm not sure you're the person to discuss it with her but DH and BM sound like doubtful candidates.  To me, that's the big issue here.

Isn't the age of consent 18 in California?  I personally think that's crazy, but it is what it is.

I would stay out of it, other than to encourage her to talk to her dad.  

Is SD telling you in hopes that you'll tell DH, and then you end up with any anger that he may have at the situation?

Honestly, because they go to high school together AND they went to prom together AND she'll be 16 next month, I don't find this to be particularly scandalous (or something that will last long-term). I DO think you need to tell SD that, while you want her to be able to come to you when she has questions (more on this in relation to disengagement in a minute) that you won't be in a position to lie to your husband. You can help her come up with what to say to him. You're willing to be present when she tells DH. But you won't lie to your DH when it comes to his daughter. It's a bad thing to do as a spouse, especially when it has to do with their minor child.

Regarding your question about disengagement, this is really up to you. I personally think that if a teenager feels comfortable sharing with an adult, that adult should do their best to help that teen. In this instance, you can establish clear boundaries with SD like I stated above. You still aren't engaging in parenting because you're not telling her what she can and can't do. You're providing advice and making it clear that her dad needs to know. 

I know teens don't want to have to tell their parents certain things, but there are times when they need to. This is one of them. If she had some sort of health issue that needed immediate attention, I wouldn't hesitate to lead her to her dad and tell her dad what is happening so he can get her help (PROVIDED her dad wouldn't make the situation worse).

It's trickier with teens, because you have to give them the freedom to make mistakes but also be flexible enough to help them muddle through the mistakes they do make. Is being 15-nearly-16 dating an 18 year old a mistake? Eh, could be. The likelihood is that they'll date through summer, at most, and split up in the fall when they go their separate ways/can't see each other everyday. I know we all jump to worst case scenario because 1) it does happen and 2) most of us are living/have lived some form of worst case scenario (which is why we're on STalk), but that doesn't mean it's guaranteed to happen. She's just as likely to get pregnant by a teenage boy closer to her age as she is this kid. She's just as likely to be in an abusive relationship, or get an STD, or have her heart broke with the 18 year old as a 16 year old.

If the real concern is consent and statutory rape laws, then all the more reason that DH needs to know and he needs to find out what the state laws are. Then a discussion needs to be had with both SD and her BF about what is and isn't allowed, and what could get BF thrown in jail. Even if your DH tells SD she can't date her BF, it likely won't stop her. So, make sure she has all the info available, including how sending nudes is still child porn even if she doesn't feel like a child.

And if DH can't have this conversation, I think it's fine to lay out the facts to SD and tell DH once that he needs to accept that he has a teenager and needs to address this accordingly, full stop. Then be done with it all.

18 year old should not be dating 15 year olds. Soon to be  16 still means 15. Why would he even want to. Hhmm He is an adult and will be out of school soon, he's going to date school girls?  I wouldn't be ok with it as a parent at all.
 

Of course if her parents don't care then what can one do besides perhaps report to authorities. Of course they'll tell you their dating consists of discussing Shakespeare and they are so respectful. Yeah. No Id not invite him for dinner. It would send a message  that you all condone adults dating minors. Not classy. Doesn't seem to be a warm and fuzzy situation of inviting SOs to meet parents. And what's up with his parents. They want to wait till she is 16? To date an adult? In a state where age of consent is 18. Not 16. So they are encouraging their son to date minors? They aren't very smart are they? 

now if he truly is a nice guy and likes her so much then I'd tell him to wait until she is also an adult.

now of course they'll not listen and do what they want but i wonder why she wants to date an adult and why he is interested in young girls.

Sadly if parents say it's ok what could you do. But I'd definitely tell DH unless she does it first asap. I'd not wait. Save yourself because if something goes sideways (and it likely will) you'll be liable for keeping the info from parents and   potentially authorities 

Despite everything, I'm happy for Munchkin.   Shes starting to have a young woman's life.