"Maybe you are." That answer was enough for him. Clearly the 'boy' didn't want anyone to know his real age. But this one would certainly need a distant eye watching over him.
Minato had seen a lot of crazy things on the battlefield. Desperate villages sending talented children to kill and die. His own village was equally guilty in that respect. A little precocious Hatake Kakashi was waiting for him back home as they spoke. Waiting to be trained at the tender age of not old enough to die.
Maybe Tomas was 18. Maybe he was 16 as he was guessing. Maybe he really was much younger.