Milopotas Beach is fantastic, as good a beach I've seen in Greece. Ios is lively, not mental but loads going on. I think the 16 year olds will have the holiday of a lifetime to be honest.

There is loads going on there. Loads of Irish & Aussies. Its the kind of place that groups of people would go to, but Im sure you will have a good time. Its a cheap island also.

It is a little difficult to answer the question objectively, I first visited the island about 25 years ago and have called in to the island on and off since then, usually stopping there a few days to change ferries as it's a minor hub in the ferry system.

The island in general is frequented by large groups/numbers of young people who only go to Ios in the Cyclades, 90% to drink and fall down, early morning sees paralytic bodies here and there, streets smelling, This is largely confined to the Chora and it's edges.

The famous Mylopotas beach isn't that great, mostly coarse sand, rocks in the sea, there are far better on other islands, and Ios itself. Most of the younger people just crowd out the 'far out' enterprises facilities about 2/3 the way along Mylopotas beach. There is a scattering of acccomodation/tavernas behind the beach.

You can find some some pleasant relatively empty beaches around the island.
Whilst the island was cheap some time ago it is now quite pricey, same as any touristy island. Despite the hi-season tourist onslaught most of the locals are quite pleasant.

I certainly would not spend two weeks on the island just nothing of interest except bars and beaches, however there are ferries to other islands so easy to get away. I usually stay in the port area which is quieter barring ferry arrivals in the night, and choose accomodation location with care.