basically from lvl 1 onwards, regarding the usefullness ( they won't be high end). I would use a T2 scale with 10str / 10dex 200max life if the total stats are higher compared to a T3 to start a climb
the Tier of an item dropped is depending on the mobs lvl.
basic mob lvl is yround your lvl (i think it's between -1 / +1, but not sure about this)
Evil Presence lvl = Mob lvl +3
Trapdoors increases Mob-lvl too (clarification needed again).
Tier 1 can dropp from regular mob lvl 1 onwards
Tier 2 can dropp from regular mob lvl lvl 4 onwards
Tier 3 can dropp from regular mob lvl lvl 9 onwards
Tier 4 can dropp from regular mob lvl lvl 14 onwards
Tier 5 can dropp from regular mob lvl lvl 19 onwards
and so on...
you will recognize the pattern after some climbs