The drawback: only works on new lifeforms. Still, that could be a marvelous gift to our children; A methuselah-like lifespan would be a tremendous gift to give your child.
Paging Stef Molyneux! This explanation actually holds water; people continue childhood behaviors long after they are productive or useful to them. And neglect tends to produce unusual levels of tantrums and chimp-outs.
See, you didn't sign your boys up to be soldiers, so we're gonna give it to people totally dependent on us -- Always count on government to discriminate in favor of those guaranteed to vote for more government.