My mom's eyebrows are much like my mom - well mannered and orderly, never a hair out of place. I don't think she ever has to do a thing to them, and yet they are perfectly arched, not too thick and not too thin.
My dad's eyebrows, on the other hand, are just as much of a reflection of who he is - unruly, sometimes faintly hostile, at times even somewhat jovial. They bristle and wiggle and pop off his face with a life of their own.
My own eyebrows lie somewhere in the middle, neither as nicely groomed as my mom's nor as wild as my dad's. I've always had to shape them and thin them out a bit, but in general I'd say my eyebrows are fairly well behaved. And yet, I've always lived with the fear that one day I'd wake up to find that they'd somehow gone rogue and gotten bushy on me. That hasn't happened so far, but I swear they are considering a coup against my eyelids. Rather than getting puffier, they are expanding their territory down my eyes. I'm finding more and more stray hairs on my actual eyelids. This is not cool. But do not fear for my eyelids! I will be vigilant. I have tweezers - girly pink Tweezerman tweezers - and I'm not afraid to use them.
They're biding their time. Until you hit 40...>;)