When i was 15, i ate alot and didn't gain weight.

When i was 16, i ate alot and didn't gain weight.

When i was 17, i ate extra alot and got fat. My key was, eat alot of rice. Basically i ate 5 meals a day. Before heading for school, a cup of milo + pies/bread. At 10:30 recess time, ate nasi lemak or nasi goreng with chicken.
Lunch time, eat ice cream or some drinks only. No money.
At 4pm reach home, ate a big bowl of mee. At 8pm ate big bowl of rice with alot of dishes.

My plate of rice is really piled up for dinner time. It boost my weight so significantly. Maybe u need to eat more before sleep.

The basic rule, calories in > calories out = weight gain. I forgot to mention, i lessen my activities during my form 5 period. I just eat and sit there play computer games most of the time. Didn't play basketball during that year.

Maybe u play sports or do alot of walking thats why so thin is it?

Dairy helps alot in gaining weight. During my weight gain period, i have 3 cups of milk a day. Full cream taste good too.