The problem is that this creates an _enormous_ incentive to have kids. Is that something you really want to incentive? Pushing it back until they're legal adults removes this.
That said, if this wouldn't be enough for someone with kids to live on, that could be a problem. Its a tough problem to solve.
Given that the birth rate has dropped below replacement level and our society is headed for Darwin's bit bucket unless we make some drastic change, the answer to your question is very much yes.
That said, if this wouldn't be enough for someone with kids to live on, that could be a problem. Its a tough problem to solve.