Lol. Didn't expect some people to get surprised about their age. It's an art style. It's pretty normal in manga/animes. Just look at "New Game!". It got a cute style which is why most people didn't think that the characters are working for a game company. Or "JJBA", where15 years old looks like a 30 years old muscle builder.
Here where I live, while is not rare see people over 1,76 , its actually pretty common to see 12 yrs old at least 1.67 cm tall, my classmate was 1.79 since 12 but the bad thing it's that he didn't grow further tho
Well, Ichigo and Chad are also 15 in Bleach, so I guess this is ok? Much better than the twig style where characters above 170cm are treated like freaks.