it's mostly down to genetics. I've been abusing my eyes so much over my life that you'd think im actively trying to get glasses, and yet my vision is still fine. on the other hand, i have friends who don't really do anything that's too overly strenuous on the eyes, and yet they already needed glasses by age 14.
seeing as his big brother also wears glasses, I'd say he just got hit with bad genetics in the eyesight department.
Honestly, reflecting off of the chapter their character dynamic really is a fun one. They're total opposites, and their interactions has helped them improve towards the other's direction.