I think there was a bit of presumptive bragadoccio being flung at us older blokes - 40 is soooo ancient! - by our young lad in the initial post. Hell, I was the same way at that age, as were we all, truth be told. To me, old guys were decrepit, never young, and just didn't have a clue. And their dicks just had to be so much smaller than my throbbing young manhood. LOL!
Speaking for myself and on behalf of the many partners I've encountered over the years who were a little long-in-the-tooth, let me assure my young friend that after puberty age is not much of a factor as regards penis size. Sure, you may think that your generation is just that much bigger because, well, you're you, after all. Duh. But you can rest assured that when you reach the ripe old age of 40, your schlong will be the same as it ever was: no bigger, no smaller; this, of course, being contingent on proper nutrition, regular exercise, and staying away from knife-wielding females of the Lorena Bobbit stripe.