Also on topic: A 55 yr. old buddy of mine with no significant cardio background - played a lot of squash and did gym stuff but was never a runner or the like - started riding two months ago. He's been riding with me two or three times a week, and getting stronger by the day.
This morning, we rode 46 relatively flat miles at an average of 18.2 mph. He has not mastered or even tried the art of drafting, so I would call it a solo effort. I'd say he has a year or more before the upside is reached, and its going to be in the low 20s.
At 64, I'm just hoping I'll be able to keep up when that happens.