Denver has a pretty huge gay community for the city's size, and has lots of gay clubs to check out.
On thursday nights, Tracks, the largest gay dance club in Denver has 'teen night' - but 21 and ups are allowed in with standard bar service. obviously a very young crowd, but since you are pretty young as well, you will fit in just fine.
Charlies, the country/western gay bar on Colfax usually has a pretty good sized crowd on Thursday nights as well.
On Friday nights, definitely check out the Compound on broadway, they pretty much have the best DJ's in town, and while its a fairly small place, the music is always good, and the dancing fun. predominantly latino/younger crowd there too.
Saturday night is the big night at Tracks, but from your post, it seems like you will be gone already