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Ok, so you know how Jason looks like he's around 19-20 when he returns, but was dead for like a year before beign resurrected by the pit?
The pit, tecnically talking, makes you younger (apart from stronger and has a healing and reviving factor), Ra's Al Ghul usually uses it for this, to be younger and stronger and to heal injuries,.
So, what if, other than reviving and healing, Jason became younger than when he died when being revived by the pit? It would make his young appearance have more sense
Even more because Dick and Jason have like a 4-5 year old age difference, and when jason return Dick is like in his late 20s (25-30) so Jason should look like he's more than 20years old, but his looks his way younger, so maybe when he was revived the pit made him younger.