What triggers a follower's armor progression?

I have noticed that there is an armor progression for each of

Kormac (Templar),

Lyndon (Scoundrel)

and Eirena (Enchantress).

What triggers the progression? Is it purely a visual effect or does it have some purpose beyond eye candy?

All images taken from http://diablo.wikia.com/wiki/Diablo_Wiki

Solution 1:

I've been wondering this myself as I noticed the change shortly after a level up, all my followers changed appearance when they hit level 28. At least they did for me.

As for any effect on combat I haven't noticed any, they seem to deal and receive the same amounts of damage as before.

Solution 2:

The follower armor appearance changes based on level. The first change happens at level 30 (I think, might be 20) and the second change happens at level 50.