I think one of the differences though between being in your thirties to being say in your late teens early twenties is that use it or lose it applies more than ever.
So if you don't train and just let yourself go by your thirties I think it really shows. but if you've been training consistantly from your early twenties into your thirties you can actually peak muscle mass wise in your thirites.
Power lifting records seem to peak in thirties (and decline every decade about hundred pounds from then on - Im talking about bench press - e.g. 700lb in thirites, 600lb in 40s, 500lb in 50s, 400lb in 60's, 300lb in 70's - maybe thats a huge generalisation and obviously I am talking about the best in the world for those particular figures - I was looking at the world records on a powerlifting web site a while back and came to that rough conclusion though - As a guy whose never benched heavier than 365lb for 3 reps myself those figures give me hope though that I may still be pushing some heavy weight even into my seventies i.e. yes there is a decline with age but not enough that a guy with top form for his age won't still be a hell of a lot stronger than your average guy of any age).
Also look at body builders at the peak of their carreer e.g. most olympia level competitos are in thirties as is current reiging champ whose about 38 or so (i.e. in properly trained individuals muscle mass also seems to peak around mid thirties - of course vitamin S plays some role in the case of Ronnie and his fellow competitors but I think that natural body builders also can peak in thirites too, In my case I know I have).