How Can U Tell If An Older Cat Or Kitten Is Peeing In The House?

Our older 16 yr old cat is still very healthy, but lately I’ve noticed him weeing more often. He’s not in any pain or uncomfortable; he just seems to be peeing more frequently than normal. My vet says it could be a matter of him getting older and needing to urinate more often. Does anyone else have experience with this? Or do you think my vet is correct that he might need more frequent trips outside?