Ride what ever you ENJOY riding, but never pass up an opportunity to ride something new or different. I've been having fun with my 29'er single speed. I also ride a Trek 7.5. The thing is it's your ride, and if you don't enjoy riding it, you won't ride it! Look around, TEST RIDE some bikes, go to different bike shops, and find one that fits. Both the bike and shop have to fit you.
And don't worry about your age. I hang around with a bunch of bike geeks, most younger, and we talk about our bikes. I mentioned I got a new 29'er SS one day. Fast forward a few weeks. We're sitting around shooting the breeze, and the topic of our ages comes up. I tell my age, 55. One of the 20 somethings sputters "B-BUT you ride a 29'er, and single speed!!" As if I was too old!
Enjoy the ride!