YZ__rider said:
A 125 motocross bike makes a poor trail bike for many reasons. If you plan on riding trails, go with a small 4 stroke such as a CRF150, or a TTR125. You may be big enough for a TTR230, XR200, or a CRF230...maybe. The only issue with the bigger bikes may be weight.
A TTR125 would be way too small for someone 5'9" tall. The TT-R125L is the big wheel version that would only be a little too small (32 inch seat height). A great bike for your first ride but it won't suit you once you want to hit the jumps.
The 230cc bikes are also good for first time rides but it isn't a bike that a 15 year old is going to want to be seen on past his first ride. They are heavy, low performance, short suspension travel (lousy for MX jumps). The electric start will be embarrassing......
The 125cc two stroke would be a great bike once you have the basics down. It is not a good bike for a first time beginner. If there is any way that you can get a few hours of experience on one of the above bikes then it could be a good bike for you.
I would recommend something like a Honda CRF150RE. It has a high performance four stroke engine, decent suspension travel, 34 inch seat height.
Rod