But because of what? 'Cause it's certainly not his height. And if we're talking intelligence, I didn't see any signs of him being mentally impaired, and he was smart enough to come up with creative uses for Harvest. If he showed signs of stupidity, then how does that compare with Okuyasu?
He's certainly creative, like a child, but he's not very intelligent, he's very simple minded, he has no social intelligence, he doesn't know the kanji in "Kasu (Borrow)" (something you learn at around 10 years old), he licks stamps to see if they taste differently, his emotional state tend to change quickly and between extremes (Happy/Angry/Crying).
See his reaction to Josuke telling him that he's doing a great thing by collecting change, or even cries when Okuyasu calls him smart because "no one ever done that before".
That's certainly not how a 14 year old acts, it's all closer to the actions of a 6-8 year old.
He does get a bit cumming during the fight with Josuke and Okuyasu, but that's due to being a character in JoJo, everyone is battle smart in this series, even babies and rats.
That's why it's hard to be mad at him for what he did Josuke and Okuyasu, it's not that he was a bad person, it's just that didn't know better.
Okuyasu is just dumb.