Usually, puberty starts between ages 8 and 13 in girls and ages 9 and 15 in boys. This wide range in ages may help explain why some of your friends still look …Signs of slope unblocked puberty include girls developing breasts and starting periods, and boys developing a larger penis and testicles, a deeper voice and a more muscular appearance. The average age for girls to start puberty is 11, while for boys the average age is 12.