They were at recess, and she had complimented how strong he was when he was climbing. He replied by explaining:
When you are new you are weak. When you grow older and get stronger. Until you get married--then you get weak again.From past conversations with Smiley I know that he is still a bit confused about how children get stronger as the grow older, but his grandparents are getting weaker and more frail. I suppose he has no concept of how old age works, so "marriage" seems the best milestone in his mind for that inflection point.
Of course, his statement is funnier because in modern American society so many couples do "let themselves go" and exercise less after marriage.