IIRC (it's been quite a while for me) the henchfolk level when your PC does but always lag behind by 2 1 levels. So when you reach level 10, for instance, your hench will become level 8 9, Their stats and equipment are pre-determined by the versions established by the campaign designers. The maximum level they can reach is 14 regardless of what level your PC attains.
edit: Almost forgot... each time you hire a different hench and your level is greater than their current level + 1 2, there will be a convo thread allowing you to "force" them to level to your PC minus 1 2 (as mentioned previously).
It could be only one level difference (yes!), but I think in the OC, it is two (no!). You'll find out pretty fast in any case. '>
edit2: corrected level offset per MM's input. Memory burp checked by one who knows better. Tx!