I don’t think it’s necessarily an age thing. Some people have more obvious hair or go through puberty earlier. I have friends who have daughters 18m - 2 years older than mine, but they are all at a very similar stage developmentally.
Some girls are not bothered about plucking but my DD really wanted to get rid of her monobrow. It takes about 2 mins for the beautician to tidy her brows up.