i had bobbed hair but my hair grew out to my shoulders i was indifferent about it. i would only recommend getting it if your hair is textured like its naturally wavy or curly or you are good at styling hair and you can always ask your stylist just make sure s/he can be trusted.
Generally I would say the longer the better. None of these girls look mom ish. I voted sexy, as long as it reaches past your chin. Also black hair is best. If your going to dye it use natural colors. You can have blue and pink highlights or tips like in that last pic. but I'm personally not fond of pic 4 on stage red hair.
They're all models, so obviously they all look sexy, and would do whatever hairstyle they had - but they'd all look better if their hair was another 6 inches longer (except maybe the Korean girl, who suits that style quite well).
Okay, I had bobbed hair for a while. My hair is only a little longer than the pictures now but it still looks attractive. I still get called hot and sexy. It's fine. This girl looks very attractive with the way her hair is now.
OH MY. GOD. There is a Chinese girl in my class with the exact same blonde hair as Hyeri. Eyebrows are the same too. I would've never known if not for this. I feel like she is less original now. I think it is fine. Looks mature but not old.
It's not "sexy", more like "cute", but you shouldn't be concerned with looking sexy while being that young anyway. No, it doesn't make girls look like lesbians or moms, it's actually the opposite. Short hair makes you look youthful. Get a bob haircut if you want to, your teen years are the perfect time to try different things and see what you like.