Why do Renaissance babies look like middle aged men ?

So you're in a museum: you see some incredible contemporary art, some bad, you see beautifully sculptures, some ugly and weird, you see some naked dudes statues and some naked ladies as well and then ... It happens. You find an extremely ugly renaissance baby.

"Why do these babies look like miniature middle aged men?" - You think to yourself.  

What happened is that these babies often are depictions of Jesus and Mary and we're influenced by the idea that Jesus was born perfectly formed and unchanged, which meant he was a middle aged baby. Thus, the existence of ugly old babies.