Why are people promiscuous?

I mean it's more usual in teens and twenties? or is it? There are plenty men and women at my work 30/40/50's who are very promiscuous and don't care who knows the details. They have been around and had shots with each other and so on for years. I wasn't too popular for not being part of it a couple of years ago when i began my job there. Never will entertain it as it's not me. Oh just in case you think I'm prudish - far from it but i think the longer and more familiar you are with a partner the better can be. I'm just surprised its the norm even among married/attached people of both es.