that's the thing; as mentioned in the last page, the kouhai meets him for the first time in that page in my hero, but if my hero takes place in the future, then they would've already met here. so it's probably just alternate timelines.
technically he could just look a lot older. so i took some textual evidence instead. when the mc meets his sister's classmate, he recalls how he looked next to his sister while wearing a uniform. that uniform indicates he was either in middle school or high school, so he would've been at least older than 10. since that was 8 years ago (the memory), he must've met the classmate when he was older than 18, or in other words, an adult.