in our attic we are planning to remodel a room that was put there by the previous owners; it rests on 2x6's and is 13' x 20', with the joists resting 16" apart (on center) - it feels very sturdy, and I only plan to beef up the corner where we plan to put a tub. (I'm not an expert by any stretch of the imagination, and I'm only going on what I've experienced personally, so it never hurts to contact a structural-engineer-type person!)
The only limitations I see are if you ever want to put recessed lights in the ceilings downstairs you'll have to go with small ones, and there is a noise factor unless you look into some type of soundproofing insulation (but maybe not, if you need to "keep tabs" on your 16-year-old's coming and going!)