Here you can find info about your hireling, how he levels, his skills, attributes etc. If you find any incorrect info, let me know.
Merc exp formula
Mercs use the same exp formula as players. However, there are some minor differences:
a) When you make the kill, your merc gets his exp as 1/3 of the base, exp/3, rounded down to an integer.
b) Mercs don't benefit from exp shrines.
c) There's a cap on how much exp mercs can get for each kill; they can't get any more than (exp needed for next level - exp needed for current level) / 64.
Also, your will not be able to hire a merc higher than your level, and if your merc ever reaches your level, he will stop getting exp until you level. Aside from that and the things mentioned above, mercs follow the same exp formula as players.
Experience Tables
The formula is level*((level+1)^2)*ExpLvl. ExpLvl value can be found in hireling.txt