My Daughter Is 3 Years Old And Her Weight Is

My daughter is 3 years old and Her weight is 12.4 Kgs. Few days back I noticed a single strand of small grey hair on her head. What could be the reason for this. Is she deficient of any vitamin? As such she is a fussy eater from the beginning and now she is sort of ok and has started to eat all food. What could be done to avoid grey hairs at this small age?