A Bayou 220 would be good for a young teen. Been looking for one for my 13 year old nephew. Just remember they aren't automatics though.

 

I have a polaris trail boss 330 and it is unstoppable in my trails. I ride mostly trails like you and i think you should get one. It is an entry atv so it would be perfect for you. So far it has lasted me 8 years.

 

One thing I haven't seen mentioned is an ATV safety class. If you haven't ridden before it's a great place to start. I'm with the others on this. Your riding conditions sound like a 250cc will be enough. Honda Recons in our area sell in the sub-2K range. They are a great quad. I even have had seat time on them. My buddy, his sons, his sons' friends, and my son all started out on a Recon. Pretty bulletproof. I think it would be a great starting point. My son and I have been riding together for about 10 years now. He still goes out with us occasionally and he's 21 now. We are even going on a trip to Marysvale, UT to ride the Paiute Trail. Will be me and my son, my best friend and two of his sons. It's a great hobby to get into.