How is jungling at 11 bad?! It frees up exp and gold for the other lanes and I'm still always the fastest leveler
It's not bad, I started Jungling amumu at Lv6 back when I started playing LoL.
The problem lies in the fact that you are leaving someone to 1v2 at top (which usually is a guaranteed loss at those levels). It also leaves you with a much smaller champ pool to choose from because without the runes and masteries, a lot of junglers struggle to actually clear the first wave of jungle creeps.
I applaud you for Jungling at those levels though.
Yes, Amumu is generally considered god-tier, due mostly to his powerful ult. If you can land his ult on most of the enemy team (and most importantly the enemy carry), you've pretty much guaranteed the teamfight in your team's favor as long as they're not all drooling morons. (which is still hard to expect of LoL's community, tho)
However, he is exceptionally prone to counterjungling due to his weak ability to duel other junglers. You won't run into that problem at your level, but once you reach the point where jungling is common, beware junglers such as Lee, Noct, Shyvana, Mundo etc. that can just walk into your jungle and take your buffs.
Also beware Lv.1 invasions. Losing your first blue is crippling as Amumu due to his high mana cost early on.
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