Followers do not gain xp and therefore do not advance. On the plus side, they are also disposable: if they die, you can just get replacements in a d4 months. On the balance, this is bizarre but fair.
The Leadership feat includes negative modifiers for those who abuse this feature.
Evil dragons with good Cha find that Leadership is not quite as promising a feat as you might think. It seems that eating the occasional follower or cohort, even the ones that clearly deserve it, makes finding good quality replacements very difficult.