Is this sort of thing common, having been diagnosed recently many years after having a tbi?

While there can be a silent period between a TBI/injury and seizures starting, older people (and children) have a higher incidence of epilepsy so unless it's correlated to damage they see, it could be from other reasons and focal/partial seizures are more likely than generalized seizures overall (different distribution of seizure types in pediatric population).

One of our kids has seizures so I knew enough after a while to not be surprised when my mom got them late in life. Seizures are more common than people seem to realize.

Hi credo,

Welcome to the forum! Just in case you didn't realize it there's an Epilepsy forum on this website. I've had epilepsy for 44 yrs. and it's not uncommon to start having seizures yrs. later after a TBI. and that's because it may have caused scar tissue on the brain. You may want to also check out the forum
Coping with Epilepsy. You may get a lot of detailed answers to your questions there also. Here's wishing you only the best and May God Bless You!

Sue