weight gain upto the mark ? How?

Question

as 21, 51 kg only
looking slim, staying good stamina
dont know what is the problem with me

Answer 1

Weight should be in proportion to your height not necessarily your age, and is heavily regulated by your culture and social eating habits. As long as you eat good foods and feel well, then you are probably doing fine. As you age, you will fill out to your adult weight.

Use this calculator to figure out your ideal weight range.
http://www.nhlbisupport.com/bmi/