Hi,

I am thin, in the meaning my skeleton is thin.
I play gym, but however I get muscles I keep thin, because my structure is thin.

Is there any way I can get robust and wide skeleton so I can look proportional.

Thank you

Unfortunately your skeleton is one part of the body that you can't change. This is the build you were born with and will have for the rest of your life.

Is it true that when I reach the 30s I may get wider?

Well yes it is slightly true you can't change your skeleton, but there's one thing that will widen your shoulder bones, swimming , a lot of swimming will make your shoulder structure wider, and then if you build the muscle on the shoulders as well you will be even wider.