You suddenly realize your childrens hair is nearly as white as your own and can't remember exactly when your hair was as white as theirs.
Also, another three old men joke.
Three old men were sitting on a bench across from a flower show. One says to the others, "you know, when I was a teenager, I would have taken off my clothes and streaked through that building. The others finally convinced him that age should not stop him so he crossed the street, hid in some bushes and removed his clothes. He trotted into the building naked and there was quite a commotion. He eventually came out, dressed and returned to the bench, his friends quizzed him about what had happened. He held up a blue ribbon and said "he had been awarded first place for "best dried arrangement!"