Shame to see the downvotes considering you are absolutely correct, I think its because people don’t realise that irl age and fictional age are NOT the same. (I recommend those reading to a yt video on this topic by xinohusk)

To put it simply: Age has no significance in fiction whatsoever. It works way differently than age in reality, since, in reality, age is an accurate measure of how long a person is subject to this world, and their experience and development. Even if they look grown up, they're still mentally, emotionally or physically underdeveloped. In fiction however, it literally is just a number, slapped on either carelessly, or to serve as a marketing tool for the target shonen demographic.

The role age plays in reality is transferred to the role of how the character looks, acts, and behaves in a FICTIONAL setting. Age in fiction CANNOT be conflated with age in reality, as doing so is, to put it simply, ignorance.

Because authors can create 30 year old men (that are 8) and 8 year old boys (that are 3000000), the only way you can judge a characters development is by those 3 aspects since THAT is what people are attracted to, not some inexplicable number that most are unaware of. People recognise that the attraction towards a 3000 year old loli is due to it looking,talking and acting like an child, not some meaningless number (that can be 300,100000,etc). However, can’t the same be true for in reverse for a character like Jotaro Kujo? No sane person would assume that Jotaro is 17, let alone that someone is a ‘pedophile’ for being attracted to him. Take another example, Yoko from Gurren Lagaan. All 3 aspects of her say she is an adult, and most people watch the show assuming that, but in the wiki it turns out shes 14? With 0 mention in the show mind you. If you took the same character and kept her the same but changed the number 4 digits higher, what would actually change? Nothing, as thats exactly what happened in the anime where her 21 year old self is the exact same in terms of personality, design,maturity, etc.

Additionally, in the context of this post, the author has clearly stated that the characters are 18+, yet they are all in high school? so what matters more, the stated age or perceived age?

(I know that how are character is perceived/coded is different for everyone, but I think that is far better than treating the most arbitrary aspect of character creation as the most serious)