I wonder why Larry left the show?
Here's something I found on a website, which should answer your question. :-)
Robert Rusty Stevens
In 1963, Rusty and his family moved to Philadelphia. He and the lady he was to marry worked for an advertising company. He wrote copy and she designed art. He later became an insurance salesman and they live in New Jersey.
Rusty didn't tell his own wife he was a child star, until a man came to their door asking about it. Jerry Mathers told the details of this story at a book signing, about a detective search that was done to find Rusty Stevens who played Larry. The detective finally located the house where he lived. He went up to the front door inquiring about the child star. His wife responded by telling the individual that he had the wrong house. She went back in and told Rusty the story. Rusty said to his wife that the detective was correct in his assumption. Flabbergasted, his wife asked, Well, why didn't you ever tell me about this? Rusty told her, it never came up in the conversation.