Not able to gain weight. Retardation?

Hi myself 33 years old man. My height is 174 cms and weight is 60 kgs. BMI- 18.5 appx. I am just on the line of underweight category. I look too much thin .I have been trying very hard to gain weight since many years but not able to get good results. It's not that I am not eating enough rather I have good appetite .even I have all the time hungry feeling. I eat though but not able to put on weight. Even there would not be any hereditary case as my younger brother is 184 cms and weighs 82 kgs. He is 9 years younger to me. But by looks the case is vice versa. I feel like since my school days, I had not grown up in anyways, still looks same. Is it growth retardation? I am desperate to gain weight so that I can look of my age. Please advise