Now I am 43 years old. Is it too old to learn to repair and change locks? What do you think of this industry?

No, you can't think like that. You are young, and some people succeed in their fifties and sixties. As long as you have this dream, you will develop and work hard towards this goal. I believe you will gain something. Since you have such an idea in your mind, don't think about how old you are. I think this business is ok. You don't care what others say, as long as you like it. Work hard by yourself, don't care what others say, have confidence in yourself, believe in yourself and believe that you will succeed. Come on, I support you!