Am online since 1989/90, can assure you most kids quickly learn how to fake their ages to sign into everything.
With home network control was able to reduce time their devices could connect to internet, now even ways around that.
Always make it clear to them IF they go on internet they can be found, traced, and dealt with, how easily they let others identify and find them will decide the level of problems they will have.
Most kids are not looking at porn, other than for a moment, because they prefer looking at other things.
IF you find young ones often looking at inappropriate detail, arrange professional counseling for them for as non-parents they more likely to listen to and learn from them.