I've just started again at exactly the same age as you, so I certainly don't think you're too old. Think about why you want to ride BMX and where/how you want to ride. If you're into dirt bikes I'm guessing you want to do some dirt jumping? You're best going to a store that stocks a decent range of bikes to see which one suits you and appeals to you. I've got a flatland bike, which is pretty heavy and small, which makes it unsuitable for street, vert or dirt, for example.