My thought is that you should ignore the number of years since your birth date and concentrate on how old you feel. I have been doing that and rode my last Markleeville Deathride (130 miles, 16,000 feet of climbing, all at high altitude at the ripe old age of 66. I attempted the Davis Double Century also and would have finished except for a disastrous mechanical failure. I expect to continue doing such things as I approach your age (coming up in 3 years!). Bear in mind that I've been riding distances pretty much continuously for 50 years so I have experience and longterm conditioning on my side. Your three centuries in three years a short two years after starting cycling is admirable. Kudos to you and keep striving for stretch goals!