Do violent video games have an effect on teens?

of course they do. but of course with parental permission & guidance given with them. threat games like movies. they also have a ratings system. in America it is called the ESRB. here is a breakdown of the system for you

ESRB rating movie rating

Ec - E G

E10+ - T PG

some T - M PG-13

A NC-17 (X)