A hard lump in the breast of an 8 1/2 year old girl may be precocious puberty or a breast lump. 1. precocious puberty: generally speaking, girls enter puberty around the age of 9 to 11 years old, and begin breast development. 8 and a half year old girls with hard lumps in their breasts may be precocious puberty. 2. Breast lumps: 8 and a half year old girls with breast cysts and other diseases may also lead to breast lumps. The onset of these diseases may be related to genetic factors, environmental factors, endocrine factors. It is recommended to take your child to the breast surgery department as soon as possible to find out what causes breast lumps in 8 and a half year old girls and actively receive treatment.