Her mane is getting quite long, and I'd like to pull it short. When should I start pulling it? I'd rather not cut it unless that would be a better idea since she's so young.
Here's a picture:
http://sphotos.ak.fbcdn.net/hphotos-ak-snc4/hs049.snc4/34821_1659607663858_1647160070_1523433_239971_n.jpg

she should be fine... you an start whenever you want. their manes dont really have growth patterns so whenever you think its too long, im sure you can just pull it

It should be fine to start pulling it whenever. Horse's hair grows basically the same throughout their life, so it really wouldn't matter if you cut or pulled it.

pulling a horses mane is very old fashioned and no show horses get their manes pulled anymore. pretty much all AQHA/APHA competitors just cut the mane if it gets to long, and then they trim the mane after they band if for a show.

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so i wouldn't pull the mane, there is no reason to, just cut it before a show, and you can use tape or a ruler to help you get a straight line.

shows AQHA

Whenever - if her mane isn't too thick then it can easily be pulled with your fingers just to take away the scraggly bits and neaten it rather than pulling it with a comb.
Yearlings manes are often pulled when preparing for sales or showing so it's fine to do.

I don't advise cutting it as it looks thick and unnatural, if need be use a thinning comb instead to neaten it if you don't want to pull it.

Also, I would disagree that people no longer pull the mane, maybe not in certain disciplines is it required but it still is for sports such as eventing, dressage, hunting, racing, showing classes etc for turn out and for plaiting.

You should start pulling it. Shes ready, it's better to get her used to it early so she learns to behave.