most multivitamin supplements marketed for pets aren't of particularly good quality and if someone is feeding a commercial diet already, many components are even unnecessary.
that whole "senior at 7" thing is something the pet food industry is brainwashing people with, to get them to buy yet another specialized food product or supplement. in reality it very much depends on the natural lifespan of a particular breed, and frankly, 7 years is far from being a senior unless a dog is of a fairly large or giant breed with a short life expactancy. a 7 year old great dane is in an entirely different condition than for example a 7 year old toy poodle.
it is also not true that one human year equals seven dog years. as puppies, dogs grow up much faster than humans do, the middle of their lifespan is fairly even, and in the end they "go downhill" much faster than people do as well.
just consider that in the first year of their life, dogs grow up to an equivalent of about 18-20 human years.
antech has a nice chart on their site:
http://www.antechdiagnostics.com/images/petOwners/dogAgeChart.gif