I started my kids on a TW125 in the fields near home around the 13 year mark.
Local farmer is into bikes so we had full access to fields which were not in use.
They learned to ride, stop, fall, race, slide, fall,and wheelie on it. (they fell a lot at first)
Bought it as a ratteler for €425 stripped the chassis and they helped paint it and put it back together.
Rebored it, put new pistons rings and gudgeon pin into it.
Had over 2 years fun on it and sold it for €600.
When they were ready for the road I put them on the Serow.