hello there.
the body weight is decided by the height of the person and not age.
calculate your BMI to find out if you are really underweight.
to gain weight the calculation is simple, you have to take in more calories than you expend in a day. and you have to take high calorie diet regularly.
have a high protein diet and start excercise in gym so that you will increase your muscle mass rather than gaining on fat.
for further details contact a good dietician in person.
regards
Answered2015-10-24 06:50:54
Let others know if this answer was helpful