Do pets need supplements?

I have cats and dogs, all of which are very well fed animals, but one of mine is getting a bit older and he's not eating as much as he should. According to my vet, if you feed your pets a high quality pet food, if they are elderly or if they are a finicky eater, then pet supplements can be helpful. I do recommend researching the product, just as you would for something you'd purchase for your own use...