I'm only a year (I think) younger than Oregon Mouse and still going fairly strong. I did the JMT, solo, in 2008 and have a 105 miler planned for the PCT this coming August. I plan to repeat the JMT in 2013; this time northbound. I have worked hard at staying fit almost all my life.

In my opinion, people are more likely to stop backpacking because they loose interest, not the ability, and house mice are largely that from choice, not necessity. Sure, stuff like arthritis takes its toll but a lot of infirmities are life-style related and can be reversed with proper diet and exercise.

So, yes, it is possible to get in at least another 20 years of active backpacking if you are only in your late 40's.