As stated in the comment above, if you mean economic life the reference you quote already proves the answer in the current environment.
The reason narrowbodies are currently kept in service longer than widebodies is due to the extraordinary demand for narrowbodies due to extreme growth of LCCs and competitors following a period of world wide deregulation. If you can't get a new aircraft you keep using the old one, even if it is not as efficient as some newer unavailable model. This environment may change at some point, and the answer will change with it.
If you mean design life, commercial aircraft are typically designed to last 30 years though this has been lower in the past. A narrowbody's life is much tougher than a widebody's so they are built tougher; they have a higher cycle capacity built into them.
A typical widebody would wear out rapidly if exposed to typical narrowbody usage. If you look at the 747D made for JAL/ANA to use on domestic routes, it not only was packed with a larger than normal number of seats, increased gross wt and reduced fuel capacity, it was also reinforced to endure the additional wear. Its design cycle life was 52,000 compared to a standard 747 at 24,000. It also had derated engines to lengthen their useful life.