I am a man. My age is around 36 years old but physically I lam looking like a collage boy of age 18 years. Why this is so ? What is the solution?

I'm not sure your problem is really a bad thing. But if you do want to look older I would say grow a beard and put on some weight. Maybe you're too skinny?

This is not a problem, in another fifteen years you will be so proud that you look younger, and in 30 years you will be bragging about it.