I've been getting AARP stuff in the mail for some years already. They start sending it when you reach a certain age, it was a bit startling to realize I'd gotten old!
I've never joined. A lot of people have sacrificed a lot more than I have for our country. I've never served in the military for example. So NOT joining AARP is just a very, very small way of helping protect the next generations.
If you're Dad served, I'd sure cut him some slack on this, He's given far more than many of us, and he deserves our thanks.