Nobody is arguing that there are late bloomers and exceptions. For feck's sake, Jesse Lingard was still called "young player" at 27
We do remember that
However, we are debating what is norm. And on average playing senior football at 21 is more of a norm than an exception. There are ton of senior footballers in top leagues that are 21, including in EPL. It is NOT abnormal or an exception to be playing senior football at 21. That is the point.
So yeah, may be Pellistri is not fully developed yet. It is a distant possibility, but there is no direct evidence of that being the problem. And by default assuming that his age is the factor doesn't make sense. He is not so young for that to be a top guess. That's all we are saying.