I feel like I've barely started the game, yet am already level 34. Some companions are capped at level 40, and others still at 30. Is there a way to unlevel myself, or cap my own levels, so that my followers don't become irrelevant due to me skyrocketing to level 60?
The Elys Uncapper mod (requires SKSE) allows you to set multipliers to your rate of skill gain in each skill, in addition to its core functionality of allowing you to raise skills above 100. I've also seen some other leveling slowdown mods on skyrimnexus, but, IMO, Elys Uncapper makes all the others obsolete.
An option I'm surprised nobody mentioned, because it doesn't require Mods, the Console, or even that you be playing on a PC, is that you can simply stop going to the "Level Up" screen. If you don't choose to Level Up, even though you accrue experience, you will stay the level you are.
The downside is that you can't govern the pace of levelling this way. It's an all or nothing method. When you do finally decide to Level Up, you will get all of the levels your experience entitles you to at once. No option to take them one at a time.
If you're playing on the PC, the UFO - Ultimate Follower Overhaul mod has a feature that removes level caps for followers, making followers be able to match your level.
It also has a feature that will easily set follower levels to match yours, if they're not currently matched.