What would be the problem, if a girl doesn't mature even after 24age?

Many reasons: You state that you are 19, but ask why one might not mature by age 24, so i assume you are asking about someone else. There are many hormonal factors that interrelate and result in breast, hair grwoth, and menses in women. It can be inadequate hormone release, end-organ unresponsiveness, etc. A thorough examination, probably by a gynecologist or gyn endrocrinologist will provide the specifics.