Being a 42 year old myself, I would say anywhere you want ! That is the fab thing about Ibiza. Was in Amnesia for Cocoon and other nights, DC10, El Divino, Pacha and all over this summer and never felt out of place for a second... Granted, I am an ex-dancer so feel in my element, but still, it's all good.
However, if you are dipping your toes in after a long break, try Pacha on any night - choose according to your music preferences. Flower Power is a blast, quite surreal to see Pacha rocking to so many old tunes !
Enjoy...