As for being Ghouls, their (Human) age has not been specified, and we do not know whether, after becoming Ghouls, their bodies remain unchanged and stop physically aging.
It is possible that their physical age becomes fixed, while their actual age continues to increase over the years without any visible physical changes. However, this is only speculation based on the fact that their bodies appear to be capable of self-repair.
Because itโs specifically referred to as a school, people sometimes misunderstand the Ghouls as being around 18โ
In reality, itโs an academy, so in some ways itโs closer to a reform school as wellโฆ
Even so, I personally assume theyโre all at least 18โ