his face only looks young because he's hardly even real and और of AN ACTUAL PUPPET. he made his body out of wood and puppetry, a new self. he's actually a puppet himself. he also now owns the 4th kazekage as one of his puppets (just like his old parents)
its because he made himself a puppet when he was the AGE 16. puppets dont age. :( serectly, somewhere i think he also WANTED people to believe/think hes younger.
he was a young looking face because he hes hardly even real, mostly AN ACTUAL PUPPET. his body he remade and seals himself inside haruko. sasori is actually older, but because of it, he looks younger, like 16. he also killed the 4th kazekage. hes now a puppet.