Is a crf150f a good trail bike for a 14 year old?

It will be a good trail bike if it's the low seat height your after.

Since this is tuned to be ridden on MX tracks the engine may bog down and quit if you just putt around the trails. That would be bad news if you're climbing hills on a slow pace.

The engine works best at the mid to top RPM. If you need run slow but need some grunt uphill, you will need to feather the clutch. That would be TMW (too much work).