I'm in Dug's camp on this one. It should be a case-by-case analysis based on the individual, regardless of age. If the driver can handle the vehicle competently (whatever vehicle, be it a steam train or a hovercraft), then age is no reason to remove them from the driver's seat.
Mistakes happen. Mistakes cause accidents - and that's what this was, clearly. Does it suck? For Disney, you betcha.
I also think these 3 bus accidents are coincidental.
And I will say this again too: These accidents show the need for a long-term plan to replace the bus system. For those who have already voiced the bus system is perfectly fine, no need to overhaul - I am not disagreeing with you in the short-to-medium-term (i.e. 5-10 years). But, for those of you who do not pre-date the opening of Caribbean Beach and TPFKaMGM, the bus system was supposed to be....(drumroll)...TEMPORARY!
I know it can't be done in a day, or for several years. But a replacement transportation system had been long overdue for WDW. It's time to develop a plan for replacing the buses (and no, not new buses), and begin work on the new system, whatever it may be.