Actually, it's not safe for any child. However, if they were going to allow lap travel for infants, they had to set a limit somewhere, and unless you'd like to start weighing kids at the airport, a hard limit based on age is the easiest to enforce.

There's actually no good rationale for allowing lap children on intercontinental flights. On domestic flights, they've actually done cost-benefit analyses. It is undoubtedly safer for a child of any age or size to be seated in an approved restraint than to be held in a parent's arms. However, what they found was that if they required infants to be in seats, some people would drive instead. And since flying is orders of magnitude safer than driving, it made more sense to let parents hold lap children than to have them drive as a way to save money.