Look it was kind of a lie and Thor should probably keep that in mind for the rest of what Spidey has said and claimed up until now. Then again, he's used to it far most than most people are one would imagine. When your brother is Loki that lack of honesty must seem downright quaint especially for how bad of a liar this youth seems to be.

"Oh, dude man. You want to try some kebabs? Or satay. I'm not sure what it is but it smells amazing and--?! H-hey!"

So when Thor assumed Youth, did he really expect one this young looking? Underneath Spider-Man's mask is a kid. It can be hard to date Earth children but he's still got the full cheeked look and the eyes slightly too big for the rest of his face. And he's jumping up to grab for the mask almsot immediately.

"I-i need that! I can't let anyone see me. I'm...I'm not supposed to be here."