RCFamily

There was was a news story of a 15 year old girl getting gang raped on Empress of the Sea's. Just because you go through security does not make these ships any more safe. 

Yes, at least there’s that!  My 20yr old googled all the ports we are stopping at and they’re all 18!  Not only that, but it turns out Antigua’s legal drinking age is 16, which is how old our youngest turned this week!  Might be funny to get him a drink, even if we don’t let him finish it ??

We were on Adventure in May 2018 and there was nothing specific for 18-20 year olds.  That can obviously change based on the Activity Manager and Cruise Director changes.  On other ships I have seen activities, but it's more of a meet at this place and time to hang out but there is no crew member that facilitates any interaction.  It's up to them to recognize others of their age and engage in conversation.  Mine are 20 & 22 now, and when they sail with us they prefer doing the adult activities.  They hang out everywhere we do, just the younger doesn't drink.  
I feel for the kids coming from places that allow drinking at 18+.  It's really not fair that they have to stop on board.  But don't forget in every port they will be legal - so drink as much as they want off ship.  LOL 

Lol, so true!!  Although, when he turns 21, I might be inclined to make him pay for his his own drink package, since he’ll be a “grown man” and all ?

One less drink package to buy.  Savor it while you can.  Once they turn 21 guess who is buying drinks for one more thirsty mouth on vacation? 
I used this to my advantage on my last cruise with her months before she turned 21.  I only had to buy one drink package.  Cruising with her now just got more expensive now that she is > 21.   

We’re bringing my 20 year old son and he’s a bit upset that he can’t enjoy a drink onboard with us... Where we live, the legal drinking age is 18, so he’s going back in time! ? We’ve been razzing him about it for weeks now. Lol

Suggest you review a cruise compass collected from previous guests to see if they offer anything for that age range.
https://www.royalcaribbeanblog.com/cruise-compass
If they do it won't be a formal program like Adventure Ocean for her age.  At this age each 20 year old is different.  Mine at that age was cool with hanging with me and going to the shows with me.  Yours may be different.  However she is an adult and the expectation is she would gravitate towards adult oriented activities onboard absent of drinking at bars.  
There isn't anything to be aware of specifically other than the common sense all adult women have to use in public.  A guy could offer her an alcoholic drink even thought that isn't allowed by policy.  At 20 some kids do drink underage at college or where ever.  While a cruise is not a dangerous place for young women there is always the possibility of a creep being on board.  Common sense rules just like it does in her hometown.