There are people of all ages there, because the island has something to offer for everybody. If you don't want to be in Mykonos Town because it can get busy and loud at some places late in the evening/night than try to find a place at Platis Gialos for instance. It's only 10 minutes away from Mykonos Town, and the bus goes every half an hour untill late in the night. A taxi costs close to zilch. There are plenty of great beaches you can go to by kaik (small fishing boates) from Platis Gialos as well. Try Elia, one of my favorites, or walk to Psarou

...I'm jealous of you already... You can check out my page on Mykonos as well if you like. Go and have a great time.