Hi Ari!
I checked out your listing out of curiousity, it's great! Makes me want to travel to Athens. It doesn't strike me as a place that would attract the younger folk.
There's a way to work around the age restriction without explicitly saying so---
1. Cost: If you're charging enough, the younger folk won't be able to afford it.
2. Make it explicit that your home isn't for partying- which, I see you have already, but maybe with more emphasis?
3. Enforce some sort of quiet time that begins at 10pm at night, for instance- for the sake of the neighbors.
The first guest we ever had was college aged. I was terrified he was going to burn the place down, but he was really quiet and totally respectful of the apartment. We haven't had any young ones requesting lately, probably because the apartment is now out of their price range. Charging cleaning fees and safety deposits will help you weed out people you don't want.
Also... if you don't instant book, you can always gauge the situation and decline. I hope you find this helpful!
Good luck! Your listing looks great.
Monta