I can think of two reasons we might have more trouble remembering jokes as we age. First of all, we tend to find remembering things in general more difficult as we age. The older we grow, the more our brain connects begin to break down. Eventually, we will find it difficult to remember even the most important details.
Another reason has to do with the importance of remembering the joke. When we are younger, there aren't as many stresses and important things to remember. As we age, we have important dates, deadlines, and other major details to remember. A joke is quickly brushed from our memory because we need to remember more important things.