Back when I first started coming to Newgrounds (01, 02), I would have said that yes - one could be too old for newgrounds. But that's just because there were so few older users, and the site itself seemed to be geared towards younger users.
Nowadays, there are hundreds of 18+ users on this site, and a large number of those are 20+, like myself. Furthermore, the site itself has matured along with its users, and it feels much more geared to 18+ users than it once was. I will be turning 22 in July, and have no thoughts of leaving this site anytime soon.
It's not really because of the BBS - don't get me wrong, I enjoy chatting with you guys. But the flash is what drew me here in the first place, and the flash is what makes me stay. The amount of talented submissions being entered into the Portal day in and day out is mind-boggling, and I don't really think I will get tired of checking em out anytime in the near future.
I just hope I can get a fast enough internet connection once I'm out of school to still be able to enjoy the site to its fullest.