It's what children start when they're the ages usually 10-18. It's the development of sexual growth.
GIRLS: Girls start earlier then boys do, there are no physical appereances shown, their ovaries are growing, and breast buds will start to show up at around the ages of 8-10.
The first real sign is breats, then pubic hair. They hit a growth stage, and grow rapidly in height. They usually have hips to carry babies. They start having monthly cycles called the menstural cycle. They're done growing quicker then boys are. Both boys and girls develop acne.
BOYS: They start later then girls. Usually around 12-18. Maybe 10-18. Everyone is different. The first notice is height. They grow taller, and their voice deepens dramatically. They start getting facial hair, and pubic hair. Their penis grows dramastically. They get muscles as well. Boys are typically taller then girls, but not always. Boys also start having sperm, and having wet dreams.
Boy: Have you started puberty?
Girl: Yes, have you?
Girl: I don't like it.
Boy: Neither do I.
Girl: Have you started your period?
Girl 2: Yeah.
Girl: I hate puberty.
Girl 2: Me too.