Health is sound state of mind and body. As long as you don't have any health illness, you are fine.
According to your height, your weight should be between 45-48kgs. To gain weight eat lot of green leafy vegetables, sprouts and pulses, fruits, exercise regularly and stay fit.
Health Tips
it is good thing that you look younger. people usually pay lakh of rupees for surgeries to look young. your face and body will mature slowly. you shouldn't be stressed for such small matters.