What does the age joke mean?

It can be understood that age has hands, learning has no knowledge at all, and joking has a burden.

Jokes have allusions. Once upon a time, there was a scholar who was almost seventy years old when his wife suddenly gave birth to a son. The son was born because he was too old, so he was named "Age". Soon, another son was born. Like a good scholar, he was named "Xue". The next year, he got another son. The scholar smiled awkwardly and said, "It's a joke to have a baby at such an old age." So he named it "Joke".

When the three sons grew up with nothing to do, the scholar let them all go into the mountains to get firewood. When they came back, the scholar asked his wife, "Who are these three sons?" The wife replied, "I am old and have no knowledge at all, but I have the burden of jokes."