For IOs with a level range, you either get a specific level or you get attuned.
If you have an IO set at level 50, the set bonuses only work down to level 47. If you have that same IO set at level 35, it works down to level 32.
If that set has level 25-50 range, you can get it at any level within that range, and it will work to 3 levels below that. If you attune them, it will work from level 22-50.
Every IO in the set slotted into the power will affect this, so if you have mixed levels of the IOs, you get varied levels for the set bonuses. Example: If you have 3 slotted from a set, one IO at 50, one at 40, one at 30, the first set bonus will work down to level 37, the 2nd set bonus down to level 47. You could attune the level 50, and then the set bonuses would work down to 37 and 27. But, then you couldn't boost the level 50 one.
Here is the wiki with detailed info:
https://paragonwiki.com/wiki/Enhancement_Set_Bonuses