I agree with Jeff, and add; I’ve always considered 10 years as the best age to start, any younger should stay with a bicycle, skateboard or maybe a mini bike, until they develop the balance and coordination to operate a motorized vehicle. Believe me when I say, kids grow up so very fast there is some wisdom to buying something they will grow into rather than out of in only one year. I started 44 years ago at age 10 with a $60 Suzuki 80 and could scarcely reach the ground for the first few rides without assistance to launch. But I rode that bike for a few years and have almost no recollection of it ever being too big. That Suzuki 80 sport K11 had 17″ wheels and weighed about 165 lbs. wet. Roughly the same weight as my current 250cc 4 stroke competition Trials bike, except K11 had a much taller seat. BTW I was always small for my age and had to do my own service and repairs from the get go.
Trials is a very ‘unique’ variety of motorcycle and the best way to locate these things used is by seeking out local clubs and competition events. That is where the dealers can be found. USA and GB has ten times the ridership that we have here in Canada.