Puberty is considered early when physical signs begin to appear at the age of seven or eight. Though this can be a variation of normal puberty in girls  and is definitely influenced by genes amongst other factors - you may want to check with your doctor if it happens before this age, as there can be medical causes for early puberty.
Delayed puberty may be occurring in girls who have no breast tissue growth by 14 years of age, or who haven't started their periods for over five years after their breasts begin to grow. Delayed puberty can be caused by medical reasons, but there are also other factors that can affect it such as low body weight or malnutrition.