How old do you have to be to go in ?

That all depends on the age restrictions of the specific show. The Greek Theatre lets each show set their own age restrictions. Check with the specific show's site or ticket provider.

It all depends on the venue.

I have seen small children enjoying shows as much as adults, great venue

Unless otherwise noted for a specific show, it is all ages

There isn't an age limit. It's an outdoor venue.

18 I think