I think girls should wax or thread it off, it's weird when they bleach it because when the sun hits your face it shows up, or if you are tanned and your skin isn't super fair the blonde shows up and makes the sideburns that much more noticeable.
Age wise i think that's up to a girl's mom, it was in my case
if i had it my way i would've been getting my eyebrows done at 13.