Showing the horse and cart 

To show one's age. When an older person makes a comment or action that refers back to times long since past and shows just how old they are, generally alienating the younger generation a bit in the process.

My grandfather made a reference to Howdy Doody today. Man, was he showing the horse and cart.

Also showing your age

Showing the horse and cart by JacintaK July 6, 2009