California motorcyclists and scooter riders ages 18 to 21, considered the high-risk group for accidents, are about to be taught a lesson.

Actually, a number of lessons.

After Jan. 1, no person under 21 will get a motorcycle or scooter license from the DMV unless he or she completes an approved training course. The current mandatory training requirement affects only riders under 18.

The new law, created through Assembly Bill 55, is specifically aimed at young riders "because all the studies show conclusively that this under-21 age group is the one most in need of motorcycle safety training," said California Highway Patrol Commissioner Maury Hannigan. The CHP estimates that about 2,000 riders will be affected in 1991.

Young riders have been targeted for training because those ages 15 to 24 are involved in the most accidents nationwide, according to statistics from the Motorcycle Safety Foundation (MSF), a national nonprofit group.

The law also creates new motorcycle license classifications: M1 for motorcycles and scooters; M2 for mopeds (motorized bicycles), motor-driven cycles and low-powered motorcycles. M1 maintains the original testing procedure for cyclists--written and operating examinations.

But M2 establishes written and driving tests for moped riders. Currently, anyone with a Class C state driver license can operate a moped.

Inexperienced riders can become injury statistics on all kinds of motorcycles and scooters, industry experts say, but some law enforcement officials are especially concerned about the extremely high-powered "bullet bikes," such as the Kawasaki Ninja, Yamaha FZR600 and Honda Hurricane.

These sport bikes, with 600 cubic-centimeter engines, are street-legal replicas of racing bikes that have a high horsepower to weight ratio. Some can reach speeds of 140 m.p.h.

"We're aware of the trend toward those bullet bikes," said the CHP's Steve Kohler. "There's a risk involved if they're being operated by young, untrained riders. The injury risk is much greater with that kind of power. There's a saying, 'There are old bikers and bold bikers, but there are no old, bold bikers.' "

In 1989, for example, drivers in the 15-to-24 age bracket had the highest accident rate in the state among motorcyclists (including scooters)--9,849 of 20,955 injuries; 248 of 620 fatalities, according to CHP statistics.

There were 845,881 licensed motorcycle drivers in the state this year, riding 643,137 registered motorcycles and scooters.