I'm pretty sure they cant be more than 1 level below you. I played a lot of coop while leveling, and my followers were never more than 1 level lower than me. you also have to take into account that they have smaller experience bars than you, meaning that while you are leveling with them (say your level 58, and they are 57) if you play the whole time solo, you are almost guaranteed that you will get to 3/4 of the way into your level, and they will level up, making you both level 58, for this short period, your follower stops gaining experience, that way they can never be 1 level above you.
However, followers do now share experience bars, meaning if you play with each follower for 1/3 of the time of a level, they will all be at half experience, but with different amounts for each. Once you achieve level 58, all your followers get bumped to 58.
P.S. I'm not sure if you level one follower up if they will all be that level, or you will have to wait till you are 1 level higher for them to get the bump.
P.P.S. I always used the Templar while leveling, even when playing the monk, who doesn't like extra healing? he lives just long enough to cast his heal spells and die