We have a goal to socialize our puppies and we do with many visitors including family extended and otherwise but the adults benefit too.
I do not think socialization should ever stop. Nor do I think you stop in teaching an old dog new tricks. There is something about stimulating the mind for us and for them that does us good later on in age.
Think about it. When you get up in age do you think you will know everything? Wouldn't you like a new hobby or something fun that you can enjoy that is new - a new challenge? Well dogs do too.
Your dog is all he can be due to his start but due to you as well! We are all in this together!