What do you do in your free time?

Well, age, then it happens spontaneously. Hehe.
No, seriously . Why do you want to gain weight?
Are you into bodybuilding or what?

Nope, I'm just too thin :3

Don't worry weight gain depends largely on your genes and your body metabolism. While you can't control the genes part you can work on tipping the scale of calorie management. So cut down on your calorie usage and consume more calories. I believe you will be able to see the results. If not, then better get your hormonal profile done things like hyperthyroidism can be the culprit at times.