Hi there, Pete from Music City USA!
Lets get you started here:
www.ibiza-spotlight.com
The site has a bounty of background info on the island from people who live there. Most of the opinions expressed on the site would be echoed by those of us who go for our visits every year.
Clubbers come in all ages from teenagers to octogenarians (seriously!). I think the majority are probably in their 20s and 30s but plenty more over 40 and beyond. Visitors to the island include people of all ages - believe it or not it is a family resort.
During the days you can enjoy the beaches, explore the towns... again, check the main Spotlight site for lots of ideas. (or the Things to Do section of the forum)
The culture is Spanish with a twist, but the island is quite international as you'll find quite a number of expats around. People are laid back and they know how to have a good time. The cuisine is a mix of Spanish and Mediterranean.
That's a few thoughts. Once you've read up the main site, feel free to come back with some more specific questions