Leadership question

[Tiri]

If a cohort is a monster that normally advances by HD, does it still do that as a cohort or gain class levels?

[atemu1234]

Unless I am mistaken, I believe they advance by class.

[General Sajaru]

Based upon this line in the DMG from the discussion of special cohorts, "If a creature in the Monster Manual doesn't have a level adjustment, this means we don’t recommend using it as either a player character or a cohort", I'd say that cohorts advance by level. However, in the table above that, it includes a Young green dragon with the note "The dragon ages but does not gain XP" which says that at the very least dragon cohorts advance by hit dice.