
Deku is 15 during the U.S.J. arc when these two meet. Calling him 16 is generous IMO.
Deku's age from the wiki:
4 (First appearance)
14 (Chapter 1-2)
15 (Chapter 3 to Final Exams Arc)
16 (Forest Training Camp Arc to Chapter 429)
24 (Last appearance)
U.S.J. Arc spans from chapters 12-21.
Final Exams Arc covers chapters 60-69, to explain when he turns 16 according to the wiki.
Also, if you haven't seen My Hero Academia then you may not understand Deku's character. Throughout the entire series he stays incredibly childish. He is the innocent and naive but brave hero. Everything about his personality is childish. It's a part of his character and it's quite likeable, I'm not being critical. The whole reason he was chosen is because he has that childlike idea of justice and what a hero should be, untainted and uncompromised by the reality that an adult would have to face.
His innocence is such an important part of his character.
He's drawn smaller than his peers and he never gets larger. He has very childish proportions, especially when compared to his peers. He never looks 16, he always looks like he's between 12 and 14 at the most.
It is impossible to view him as anything but a child when you watch the anime or read the manga.
like if you find Icarus’ credit image problematic, dislike if not.