I could see a problem with people under 21 "being offered alcohol" even if it is only virtual(as people tend not to think hey what if this person is under 21). However I personally don't have a problem with it(as someone under 21) as it's not real and I can reject the virtual alcohol.
However I've also had lots of experience online having to tell people hey wait I'm underage don't say that to me (before I was 18). However, not everyone is used or wants to do that. I know some people that would be uncomfortable with illegal stuff even if it's pretend (getting offered alcohol while under 21, other people under 21 drinking, etc). Drugs that are illegal regardless of age would have similar issues, just effecting more people than the 18-21 population.
Just some things to consider.