19 year old cat throwing up

I have a 19 year old cat. He has been throwing up almost every day since we moved into our apartment in June. I put him on a Purina One cat food for Senior cats and it has helped some i think but he does sneak my other cats food once in awhile. So I was wanting to know if this throwing up is just a case of poor digestion from being so old or if it could be something else causing it?