In Florida, we tend to have more than our fair share of elderly drivers. It definitely causes problems.
About a year ago, an elderly driver put his car in reverse and hit his gas by mistake, causing his car to speed backwards... into a group of people leaving church, injuring them all.
I witnessed a woman hit her gas instead of the brakes and she hit the gas hard enough that her car went over the parking stop and into a light pole. She was okay. The pole wasn't. On the bright side, had it not been for that pole, she would've launched herself into oncoming traffic. I'm pretty sure that pole saved her life.
I have seen many, many situations where the elderly are going at least 10mph under the posted speed limit and causing severe traffic problems and many near-accidents. My favorite is when there's more than one and they are in multiple lanes, preventing everyone from getting by.
More than once, I've been driving the posted speed limit, only to have an elderly person, moving at a much slower rate of speed, change to my lane, with no signal, nearly causing a wreck.
When I worked in retail, I would have elderly customers complain about the credit card machines, citing that they are "legally blind" then getting in their cars in driving away.
While I think every demographic has its bad drivers and not all elderly are bad at driving... I still feel like the elderly win for the worst drivers.