My two cats are 14 and 17 and won't eat the Whiskas 11+ stuff (which is grim looking). I do feed them 7+ or other senior food now chiefly to keep weight on Campbell who has always been very trim but is finally starting to show her age. Don't think it's any more filling as Campbell is always claiming to be half starved despite a limitless additional supply of kitty biscuits
According to my vet when I asked the same question the main difference is to do with vitamins and weight watching. At the time I had kittens that would eat the old cats food and an old cat who would eat the kittens food!