The normal age for beginning sexual development considered: for girls 8 — 10 years and for boys 10 — 12 years. If start puberty earlier this age, we can talk about premature sexual development. Sometimes it can be a normal variant, sometimes a symptom of certain diseases in a child: it is determined by the doctor.
The same applies to the delay sexual development. If to 15 — 16 years for girls and 16 — 17 years in boys is not relevant changes occur, you should consult a professional to identify the cause of delayed puberty and possibly prescribe treatment.