For p2p there's no point staying that low, you could stay 41 and have some fun, but you need some kind of special attack to be really successful. I had fun at 50 on my pure for a while, but in my opinion 60 is where you'll start having some real fun (and probably getting the most kills).
f2p obviously if you're going to be fighting other pures you may as well stay at 41, but you'll probably find yourself fighting people in rune a lot, and 41 simply doesn't hit often enough to successfully PK in my experience.
Totally up to how you prefer to PK, but if number of PKs is all you care about, 60 is a must in my opinion (besides the blitz-maul combo, which is pretty boring after a while imo).