Males normally leave the group when they reach 11 years of age and start their own group when they hit 15 years or so. Mountain Gorillas don't become ready to have babies until around their teens. The females become sexually active around 10 years old and males start around 12 to 15 years old. Females are only able to conceive 3 or 4 out of each month. After giving birth, females aren't able to have kids for another several years.
Females only give birth to a single young at a time.
Females only give birth to one surviving baby every 8 years. Females have about 2 to 6 in a 40 year period. Males have a harem of 3 to 4 females, so, he fathers about 10 to 20 young over 50 years. Baby gorillas weight about 4 pounds at birth. They mature much faster than human babies though.