Welcome! There are at least 2 of us in our 70's here and many more in their 60's. IMHO, you're not too old until you can no longer put one foot in front of the other! I had to pretty much quit backpacking in the late 1980's after I wrecked a knee X-C skiing and could no longer carry a 40 lb. backpack. That pretty much limited me to car camping and dayhiking. It wasn't until I discovered lightweight gear about 7 years ago (with the help of this site!) that I found that I could get along just fine carrying 20 lbs. instead of 40!
That being said, you can make things easier by (1) getting yourself into condition, which primarily involves walking plus maybe some strength training and (2) using lightweight gear. For the second, see the articles on the home page of this site, left hand column,
The low back issues can be improved by (1) exercise, particularly strengthening the "core" muscles that support your spine and (2) a properly fitting pack. It's generally recommended to get your pack last and take all your gear with you when getting fitted.
Another tip is to try trekking poles, which turn walking from a lower-body exercise into a whole-body exercise (called "Nordic walking") that exercises those core muscles. Here's a primer on how to use them: Pete's Poles Page
On sidewalks and such you will want to use rubber tips on the poles, which can be removed for trail hiking.
I'd also consult your physician before getting involved with anything more than gentle walking, just to make sure you aren't making matters worse. As with any exercise, start slowly and gently and gradually work up. I'm doing that right now after being laid up for several months with severe plantar fasciitis (don't neglect stretching those achilles tendons!).
I'd also suggest googling for any hiking or backpacking groups in your area. Day hiking with others is a good way to start out. I'm sure your family will feel better if you go with groups to begin with.
Try to get your wife interested, too--find some place with gentle walking (you need to start out that way, too) and great scenery.