Hi,
Every person is different. Healthy children get puberty early as compare to underweight children. So don't worry .
Secondly regarding underweight , you can contact me directly . So that I can consult him and accordingly I will recommend diet which not only help in increasing weight but also help in achieving puberty.
Next Steps
For more information and to contact me directly visit https://www.facebook.com/pg/doctorplanmydiet/about/
Thanks with regards
Dt.Ankita