My suggestion is not to go with any particular age really.
Look for indicators that will tell you when it's time for the individual girl to begin shaving.
A few that come right to mind are:
1) When you go to the zoo and you have to show the girl's birth certificate and photo I.D. to leave , you know it's getting close to that time.
2) When you go camping and your little girl is at the BACK of the pack of running, screaming campers, and she's yelling " What's a Yetti ?? Why is everyone running from me ?? Wait for me!!".
3) When her Orthopaedist tells you that unless she begins shaving, the added weight of all the beads from her AND all her friends bracelet making kits, combined with all the 7th and 8th grade girl's field hockey team's hair barrettes may cause permanent damage if not removed soon. These are not to be used to disguise hairy legs.
4) When at a glance, you can't seem to see the divide between what's dog and what's her lap when she's snuggling with Fluffy on the couch, watching Planet of the Apes , over and over and over again.
5) When your landscaper offers to give her a quick trim while he's still got the equipment out after doing your hedges. Do not pay more than $25 for this extra service. You can probably get the guy that operates the weed wacker to do it for less if you toss him a few bucks on the sly.
6) When anyone exclaims "Holy sh*t , it's moving towards us!!!!" , it's a true sign she's become the freak with the overly hairy legs.
(added late)
7) When she gets out of the pool and her legs have better grain patterns than some of the finest imported shotgun stocks you've seen.
8) When she gets repeated offers to join an Alaskan dog sledding team, with her pick of possition up front.
9) When she wears stockings and it looks like a Chia pet experiment.
10) When she wears a skirt and it looks like Cousin It from the Adams Family is giving her a piggy-back ride.
These are just a few that come to mind. Other signs will present themselves if you watch carefully.