At 64, I've done 3 centuries over the last couple of years. What I have learned is that it all depends. How basic is that? Vertical means a LOT.
My first century, 102 miles had 11,000 feet of climbing. Took me 9 hours. Pure sufferfest. The last 25 miles had almost 6k of climbing. Not a "fun" day.
No 2 was a flat century in Savannah. 102 miles and it took me 5:20. It had like 500 ft of "climbing". Going over the bay bridge was the highlight of the day.
No 3 was the Horrible Hundred in Florida. Again, same mileage with 4,500 ft of climbing. Yes, this is the very hilly part of Florida if you can believe it. That one took me 6:20. Very pitching climbs. Not long but I counted about 10 sections in double digits. Only ride I was ever in where people were actually layed out on the side of road dead to the wind. Obviously, they had not climbed very much.
So it all depends on the climbing. Doing any century in 5 hours is moving which means you are averaging 20mph plus. Not easy when you get over 50 by any stretch. Add in some climbing and for most people over 50, forget about 5 hours unless you are dead flat. If in a group, with some help, you might get there. Hills, no way IMO. Now, I am done with the big climbs. I'm ready for some flat stuff.
john