I would highly doubt old age was the reason if you buy a clownfish thinking it's 1 year and it dies...they can live easily over 20 years, many over 30 years. If you're getting a clownfish of that age, chances are you will be able to tell.
The chances that you bought this clownfish one year before it dies is unlikely(possible, but not normal). Most clowns in the wild will not live as long as in captivity...nature of the beast. They get old, their offspring no longer viable or healthy as before, pushed out of the anem by a more aggressive youngster and is eaten. Not necessarily every time, but much more likely. I think a clown's potential for life is much higher in a proper tank and if you get them wild caught and take proper care to rid diseases, parasites, etc, you should not have a loss in the first year. I doubt there are many 20+ year old clownfish in the wild, but they all have that potential in captivity(barring bad owner, bad tank, or problems coming in explained above).