Think about the depictions of the Madonna and Child painted before the High Renaissance; during the medieval period and the early Renaissance, for example. I would venture to say that ever baby Jesus painted around this time appears to be on the wrong side of 40.
This baby Jesus – painted by Paolo Veneziano in 1333 – is definitely a racketeer.
And this baby Jesus – painted by Duccio di Buoninsegna in 1283 – looks like he's already owned a butcher's shop for 25 years.
So why is this the case? Medieval and early renaissance artists very evidently knew how to paint people. Take Giotto's "Lamentation," for example, painted in 1306 – one of the most beautiful depictions of unbridled emotion I have ever seen.
So if Medieval and Early Renaissance artists DID know how to paint, how come they seem so incapable of drawing a baby who doesn't look like a middle-aged man?
The fact is, it's intentional. There is actually a rather straightforward theological reason for these depictions of a Baby Jesus with male pattern baldness.
The Blessed Trinity, one God, is perfect and unchanging. Jesus, therefore, being the second Person of the Blessed Trinity, shares this attribute. Medieval and Early Renaissance artists believed that Jesus' body ought to reflect his perfection and changelessness, and the way in which they sought to achieve this was by depicting the Baby Jesus as a "homunculus" – a "little man." They did not wish to portray Christ as a normal baby who would develop into a man. They wished to portray him as having been born already fully developed.
For your edification, here are more Medieval and Early Renaissance paintings of the Madonna and Child:
And here is how the conventional depictions of the Baby Jesus changed during the High Renaissance:
