Ryan hit it right on, so I'll just give you a personal annectdote.
After finishing college where I ran 1500 – 5000 on the track and xc, like many of the day who kept running I took to the roads with the focus on 10K and up. For the next 4 years or so I only ran a couple of 5Ks (with no speedwork) and mostly did 8-10K pace work for interval days. Then I got injured and was out for almost year, and took almost another year to get back. So at 27 I decided that I missed doing track and started doing all comer meets again, including some 4X400 relays for fun. I was never blessed with blazing speed or quick acceleration, my splits were quite close to quite close to anything I did in college (~55s), and I PR'd for 800 at 28, as well as 5K and 1500.
But yes, as you get older the speed isn't there anymore. I couldn't break 60 if my life depended on it, and 64 or 65 would be real tough to match anymore.
(need to post here more, been hibernating all winter!)