I don't think it ever becomes necessarily "innapropriate" .  There are lots of factors.

If the family has ample means and takes their adult kids with on vacations.. because they enjoy being with their kids.. vice versa and parents can well afford it?  I see no problem.

I think it's probably less typical that adult kids take ALL vacations with their families.. but I can see treating your kids to things if you enjoy them.

Generally once kids move on and it's more than just them.. ie a significant other or family.. and they are working.. the kids should to an extent start picking up the tab for at least some of their travel.

We still go with my YSD a few times a year.. we pick up some of the cost, but like on our most recent cruise, she paid for her own single cabin herself... though we did gift her with some excursions in honor of her Bday and Christmas... so we could all have fun together.

I think there are a lot of factors that come into play.

My parents rented a lake house a couple of years ago and wanted all their kids and grandkids to come spend a long weekend. I contributed some of the groceries for the weekend but that was about it (I am in my 30's)

We have also done family cruises where it is fend for yourselves- that started when I was 20. If we wanted to go, we paid for everything ourselves.

We used to do a big 12 bedroom beach house every year. We paid a % based on how many bedrooms our household wanted in the house. I miss those summers! They were always the best vacations! We can't coordinate my whole family's schedule anymore

None of my kids are old enough for this to be a consideration yet, just kind of basing it on how my family does things and I think it is pretty fair. 

Around 18 and above, I think if you can afford to take them and want them with you, why not? But they should pay for their own food and shopping, of course. 

I've gone on cruises with my mom and stepdad twice since turning 18 and I paid my own way, but they paid for our flight to the port because they had extra points and wanted to help out. 

I'm going to Ireland with my dad and stepmom at some point, and we plan on paying our own way (flights and hotel) but I'm sure my dad won't let me pay for dinners out and such because that's just the way he is. 

When my kids are 18+, I will pay for PART of their vacation with us and they can pay some of their own (IE I'll pay the flight, they pay for hotel, whatever) just because they're less established and I'd want them to enjoy without stressing the whole time. 

ETA: We also go from ontario to new brunswick every summer to visit my dad. Last summer, I invited my sister and brother (and their kids/SO's) and offered to pay for the cottage for all of us. My husband and I usually spend $800 or so for a week and it's hard to find anything smaller than a 2 bedroom, so we spent $1000 on a 3 bedroom because why not? I'm in my early twenties, my sister is 26 and my brother is 30. They don't make as much money as my husband and I, so I wanted them to feel included because I knew they couldn't afford to go visit my dad if they wanted to. They paid their own gas/flights with help from my dad. 

I think whatever is appropriate FOR YOU. I don’t believe there’s rules when it comes to that. If you are comfortable with them on your vacation - awesome, if your not - awesome too. If you feel like you and/or dh can pay for them without making it a big deal go ahead. If you feel like it’s something that would create some sort of discomfort then don’t. 

If your kids/skids are adults it’s time for your “wants”. 

I think what is appropriate can vary by family and their means, closeness, etc.  A lot could depend upon who wants/plans the vacation.

My parents have three adult children.  They take us on a family vacation at least once a year, usually over the Christmas holiday, and make their vacation house available to us at other times.  They WANT to vacation with us, and they pay for us to go.  We could not otherwise afford to accompany them on these vacations, or would choose to spend our limited vacation money on different vacations.  My parents are quite wealthy.  We kids are not.  I don't think this is inappropriate.  Now, if my parents were going on a romantic getaway, I think it would be inappropriate for me to ask to go along, regardless of whom was paying.

Taking adult SK on a family vacation where parents,can afford to paid for it.?  I don’t know what’s in people’s pockets, If they can afford it that all well

Taking Daddy daughter trip where SM is Not invited. NO..... no matter who is paying for it. 

Taking adults on family vacation if parents can not afford it. NO, kids have to chip in so it really works cost wise 

not taking adult SK on vacation at all.  No one has the right to free vacations. There are adults and can do what they want 

I went on many vacations paid for by my parents, well into my adulthood. My mother still pays for a rental house for all of us every summer.  I don't know of any families where 18 year olds suddenly have to pay to go on vacation with their parents.

If parents have the means and want to take their adult kids on vacation, that's fine, IMO.  When people get divorced, it is upsetting to some stepparents if the bio parent wants to pay for their kids to go.  I don't really get that.

I think it depends on the family dynamic. My parents took us on vacations until we graduated from HS and either went to college or started working full-time. At that time, we were expected to pay our way (FT) or pay for our souveniers (college).

I went on annual vacations with my parents (2x a year) because my Mom had Alzheimer's and my Dad needed help. However, I always paid for myself (unless Dad INSISTED - meals). I did that up until my Mom passed away 7 years ago.

takes the grown skids on their vacays at the Outer Banks.  Most recently a snapshot of OSS22 grinning from ear to ear looking everybit the East European elderly woman in a babushka.