I know this is all light-hearted banter but my first thought is (take into account that I'm a novice, the most I've every ridden in a day is 45 miles) that is would be more realistic to ride the 100 miles at 50 years old and work down (a mile a year) to 50 miles at 100 years old. In doing it this way you acknowledge/accept that age does affect performance but doing the 50 miles at age 100 would still be quite an accomplishment.