Wonder how long he's known something was up with this? I can see many people his age knowing and just keeping their mouth shut.
There's no telling, but a lot of times tumors are asymptomatic, unfortunately. The seizure may have been the first symptom. People usually don't get brain scans as part of a regular check up. Kennedy indulged heavily, and probably didn't feel good, anyway, so he had probably learned to tune out pain from hangovers, etc.