I will be 44 in October, so not to far off from your dad. My family used to do "over the hill" birthday parties for each decade starting at age 40. Then there was to many of us at that age and it stopped. Sometimes the party was set up all in black, almost like a funeral, and sometimes it was set up like a baby shower with pink and blue balloons, and all the other baby paraphernalia. I am down to 1 cousin who is not, legally or physically, older than 40, I have several cousins and a mother who are celebrating anniversaries of turning 35, but won't claim to be older than 35.
I guess my answer is, make it a joke.