Hi Monique!
If you ask me, the place to go is definitely "Joy Eslava" in Calle Arenal, It´s just next to Puerta del Sol. I´ve been there with my parents (I´m 29 so figure out how old they are) and we´ve always had a great time. At "Joy" you can find twenty-somethings as well as people up to ??
The nice thing about this club is that it´s one of those places where you
a) can show up any weekday, there´s really something going on every night
b) has been trendy for about 20 years now
c) offers the possibility to go up to the 2nd or 3rd floor, sit down at a table and just listen to the music (disco,salsa, etc.) or watch the crowd dance on the dance floor.
You sure would not feel out of place here -even being +40
Have fun!
Saludos