Vitamin C production is only part of the equation. The more nutrients you take in the form of vitamins and minerals, the longer you will live, weather you're a mouse or a human.
Animals, like humans, absorb less as they age, and with less absorption comes a weaker immune system.
If vitamin C was the fountain of youth, Linus Pauling would have lived forever. The clock starts ticking the minute you are born and nobody lives forever. The best you can do is to slow down the process as best you can by avoiding the land mines, (drugs, alcohol, smoking, obesity, etc) taking in optimal nutrients, having a positive attitude, exercising, and being happy with your life.
Vitamin C is just one spoke in a large wheel.