Older cats vomit more often. They often do it because their kidneys and/or liver don't work as well. Also it could be a glucose issue. Older cats are also more prone to stomach ulcers or just stomach distress. You could check with your vet if they have a cheap blood test set for elderly cats. Ours does. It includes blood cells, sugar level, liver function and kidney function. If you have a cat with aging liver or kidneys, there is special feed for that (a bit pricey). But if the tests are fine, it can be that the older cat just has a sensitive stomach.

Mine are middle aged to elderly and they both vomit now and then because their stomachs are becoming sensitive to the quite fatty food I feed them. Problem is they refuse eating other stuff. I try to find other foods but it is not easy.