53 can group with 80 for example.
Just be careful. If you kill grey-to-the-level-80-character mobs, you can still get regular xp but you can't get AA xp on the low level charcaters.
In order to get AA xp on the low level character, you must kill at least green to the highest level character.
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After 16 years, I'm not listing every friggin character.