Puberty is started by hormones, which are chemicals that tell parts of the body to do things. Today, on average, girls start puberty at age 10 or 11, and boys at age 11 or 12. However, people all over the world start puberty at different ages. Changes that happen to boys and girls during puberty include the sex organs growing bigger, hair appearing on the body, and growing taller and stronger. These changes are called secondary sex characteristics. People often notice boys' voices getting deeper, and girls growing breasts and starting to menstruate (have periods).