"It's okay! You don't need to apologize that much..."

S-4 certainly notices the change in Frisk's age, and he's frozen in shock until he hears the mention of Papyrus.

"Oh... you're new here, aren't you? I'm not actually Papyrus. Not really... I guess I'm from your past. You can call me S-4, f-for Sans-Serif Support System."