Yes, retainers can reach level 60. Remember that the maximum level of your retainer is limited by the level you have reached on your character for that job. If you're only maxing a single job, then make sure your retainers are that job.

I used to have an Archer, Gladiator and Conjurer retainer as well as miner (they were all 50). I've since reset the Gladiator and Conjurer to be another Archer and a Botanist. I expect I will always have those jobs maxed out.