A little bit more info: as others have pointed out, many of the young guys you'll see in Manhattan don't live here - they live in Queens or Jersey or elsewhere. Those who do live here have either had rent-controlled apartments for many years or bought apartments back in the 90's when real estate values were depressed - probably a bit older than you're looking for. The young guys who DO live here generally have roommates and very small places.

On the other hand, the young guys who make good money and can afford Manhattan work a lot so it can be hard to just sort of run into them. But David Barton gym is probably a good start. As to clubs, just pick up the HX or Next and look at the listings. There's a good cross-section of guys at all the big clubs.