In Batman: The Resurrection of Ra's al Ghul, Batman is healed from a stab wound by using the "Fountain of Essence" (a variation of the Lazarus pit). He is healed to full health and slightly de-aged. It's not hard to imagine that such a situation would add a couple decades of vitality to his crime-fighting career. Granted, it's from a comic, but it's not hard to imagine something similar happening in the DCAU.
I'm sure there were other situations in which Batman was healed from the brink of death by some mystical or technological marvel. Every one of them could have added a few more years to the old guy's career, and that's not even taking into account the possibility that he might have sought out ways to restore his health.
I might be wrong but all of the Justice League members were turned into kids in the first season of JL. Its not unreasonable to suggest that they might have come back from that a little younger than where they started.