There are ways to write a phenomenally powerful and insane cosmic god hiding in a random human's body, and make it entertaining. But this isn't one of them. The writing style comes off as very amateurish, so the comedy misses most of the time, and the way characters talk or think reveals a questionable level of reasoning. And that's not mentioning the rather unsatisfying payoffs. I encourage the writer to look at a few better examples for whimsical main characters before continuing! I like the concept behind this and would like to see improvements.

There is no evidence of this being a common thing in antiquity. Its not uncommon to have teens and adults (15 and 30), but ancient people arent stupid and know a young person pregnant at that age would die giving birth and probably miscarry.