Ojisan age bracket can be pretty wide, but 81 sounds to me more like an ojiichan. That said, if they're running a business and are still somewhat lively they ca

Ojisan age bracket can be pretty wide, but 81 sounds to me more like an ojiichan. That said, if they're running a business and are still somewhat lively they can possibly still pass as ojisan in context. Oniisan is much younger usually; usually I don't hear ojiisan so much as ojiichan (as well as ojisan rather than ojichan), my wife is Japanese and it rubs her weird to hear the opposite honorific in these cases.