Hello! I moved here when i graduated from Uni becuase it was handy for work at the time. I am now 26, have moved jobs numerous times, but have never left forest hill. I now have a lot of friends aged 20ish - 30ish who all live in forest hill. It's a great place to nurse a hangover over good coffee on a saturday morning, hang out in the horniman park on a warm afternoon, and meet for drinks at any of the various lively pubs around the station.
There is live comedy, quizzes and bands at the Hob directly opposite the station, and lots of great nights at the Sylvan post. And for really good really cheap pizza try Question bar.
And if all that's not enough, it's an easy train journey up to east london for the clubs!

As someone else suggested, go for a drink at the Sylvan post any evening and see if you like the atmosphere!