You gotta weight the pro-con. I didn't get fire control until I hit 51.

Level 36 - 41
Pro: Typically easier kill, less likely that you will lose your weapon, no need to massively go abs to get kills through iceball spam
Con: No iron grip (unless level 40), likely no barrier (deathblow/wrath still hurts), chances of getting more than 1 charge, if that, per kill is low, less people to pk (especially based on the playerbase).

Don't even consider level 42-50 - you get passed level 41, you powerrank to hero. No discussion.

Level 51
Pro: Lots more people to kill, you get barrier (I didn't find my sleek black until WAY later - I used the 4 or 5 limited black sources that I knew and never had a prolem with barrier), when you kill people more charges, typically better equipment
Con: Must rely on a/b/s (for the naysayers, I don't care - you go abs or you delete your character as an AP), more badasses that can kill you, more roving ganks, healers annoy the crap out of you, STSF, legacies, characters with rewards, deathblow does unspeakable

I went past 41 because I didn't have anyone to kill. I expected people like Voralia and Gilut would have 5-6 charges, but alas, wasn't meant to be. But regardless, the amount of charges I accrued at level 51 was worth it.

And Spear? Really? Cmon, only thing spear is good for is vaulting. And you should only vault when you have them slept. And chances of sleeping them at level 51 is pretty low. Not to mention you don't have the ability to crippling strike or lashlag them