The article doesn't say that older riders have more accidents per capita. It says that there are more older riders, so the issue is more visible. It says they get injured more. Those are all different things.
I am amused (and annoyed) by the statement in the article that
Another theory is that older riders tend to ride bigger bikes, “which may be more likely to roll or turn over,” Ms. Jackson said.
It shows a complete lack of knowledge on said researcher's part about motorcycles and riding, and causes me to question any conclusions she may have reached. Bigger bikes may indeed be a factor, but thinking that "rolling or turning over" is a cause is just ignorance.
(Plus my own pet peeve is triggered of calling that a "theory", when it doesn't even qualify as a hypothesis, much less an actual theory. A theory is something that has been thoroughly investigated and a high level of support, and is very likely to be true. A half-assed hunch is not a theory.)