baby man child

It's a baby that looks like an old man. Or an adolescent male who attempts to act like he is grown but in turn appears even younger than he is.

Google Image "baby jesus baroque paintings" and you'll see

(ex. 1)
Ben: Did you see that baby over there? Totally looks like an old man!
Jerry: That's a baby man child!

(ex. 2)
Amy: That freshman over there thinks he's so cool, he's bragging about how many girls he got with last night.
Caleb: It makes him seem like he's still in middle school, such a baby.
Cara: That makes him a baby man child!