How old should you be to drive a yz 125?

There is no single answer that fits this question. The answer depends on the ability of each person. If I had to pick the best general answer, I would say at least 14.

A Yamaha YZ125 is an extremely fast and dangerous cycle. And no matter what the age, the rider must wear ALL safety gear. Also, there are no rear footpegs for a passenger so don't EVER ride someone on the back. Many people have died or have been seriously injured (like PARALYZED) not following all safety guidelines. Trust me, these bikes are deceptively fast.