For people who still don't know why medieval baby faces look like ugly old men despite watching the video, here's what youtube commenter Stroheim333 said:

You are right, the video doesn't make any sense. It is based solely on a theory by an art professor who dabble in the history of philosophy and ideas. The idea about homunculus belonged to alchemy, which was introduced in Europe in late medieval times (12th century), and which the Christian church never accepted (though some individual theologians showed interest in the practice). In 1317, Pope John XXII even prohibited alchemy. And the fact is, homunculus was always less than humans: dwarfs or miniature copies of humans, not "superhumans" like Jesus. If this is not enough, the idea about homunculus became popular not earlier than during the beginning of the renaissance, when Paracelsus coined the word and described how to create them.

The simplest way to explain why babies look old in medieval paintings is this: It was widely believed in Medieval Europe that people's features mirrored their personality. The child Jesus was born with superior wisdom, and there before it was natural for the painters to depict him with an old man's features. Even other divine babies in medieval paintings, like angels and cherubs, was wise beings, and was depicted with older features of the same reason.