Age limit is 18, for sure.

A great suggestion if you come to Madrid is "69 pétalos club". 70-80's decor, and renewed 80's-90's music, go-gos and performance all night long, starting at 2 a.m. The dj is a well known radio presenter, also co-owner of the disco.
It's always crowded, age between twenty to forty something...
I love this place because it's very original. It's near the Santiago Bernabeu stadium.
http://www.grupo69petalos.com/home.php

Another suggestion is Berlin Cabaret, cool place, 1-2 performances per night (flamenco shows, drag queens, etc), 80's-90's music (at least when I went there some time ago). This is near La Latina area. There's a program on their website.
http://www.berlincabaret.com/

Options are unlimited in Madrid.
FIESTA!! smile