Is it possible that half demon children might mature quicker than human children? I havent read the books, so apologies if the idea contradicts anything, but rapid maturity is often used in sci-fi/fantasy, so Capcom could have Nero as Virgils son if they really wanted to. It would hardly be the most outrageous or unbelievable thing to happen in a DMC game would it?
I'm not saying he is or isnt, but I dont think you can use simple math and human growth patterns to say "its impossible", not in a universe as strange as the DMC one anyway.