We can expect a portrait of Jesus, from either the Roman or the medieval eras, to look something approximating this:
(6th century Byzantine depiction sourced from Wikimedia Commons)
However, in medieval Madonnas that focused on a more abstract form of depiction, baby Jesus (his head is intentionally depicted as that of an adult male, i.e. Jesus is depicted as a homunculus) looked like this:
(Salus Populi Romani from the 13th century sourced from Wikimedia Commons)
Why is there such a significant difference? I've noticed that depictions of Jesus can vary a fair bit in general, whereas medieval abstract Madonnas tend to be very uniform (minus the late medieval proto-Renaissance depictions which focus on realism more than abstraction). In some of these Madonnas, Jesus is even shown to be balding. How can I understand this discrepancy?
EDIT: Here is the basic difference I'm referring to: why is the baby Jesus depicted so much older? The first image clearly depicts a man in his late 20s/early 30s, with long hair and so forth. The child, however, tends to be depicted as middle-aged, seemingly in his 40s or 50s, and many depictions even show him as balding. What's the reason for making the child middle-aged rather than like the early 30s Jesus we're used to?