Short answer: No, you're not too old. I'm 72, and still backpack; I'm also diabetic, controlled with prescription meds (no insulin) - the exercise actually helps control it.

However: I can't backpack all the places I used to. My "limber" is deserting me, so I can't do the steeper slopes or stepping up or down from one rock or tree root to another. My stamina isn't up for 12 mile days anymore; 6 is the norm, 8 in a pinch. I recently discovered I can't backpack Shenandoah NP anymore - too rugged for me with a pack. But I can day-hike, and make the last day hike a short one, on an easy trail, with my full pack so I can camp a mile or two from the road. I've been to some neat places (including a couple sections of the AT), but now find my backpacking scaled back to state and local parks in Southwest Ohio (my home - with a 26 mile overnight trail a 15-minute drive away.) The scenery isn't as spectacular, but I still get a great deal of enjoyment watching the seasons change and sleeping outside after a pleasantly tiring day of hiking. I intend to continue doing this as long as I'm able, at least into my 80s.

Solo? My wife's not thrilled, but she's OK with my going solo on the local trail since there's cell coverage (I can call 911 if I get hurt, and check in with her each evening from the comfort of my tent.) Like you, I can't find people to go with me regularly (the one guy who would moved away a couple of years ago.) If solo isn't a possibility, look into volunteering with a hiking group, like a Scout troop.

My hiking is now limited to Ohio, Indiana, and the central plateau of Kentucky. However, there are lots of places in the Northeast: the AT between Maine and, say, New York State leaps to mind. The White Mountains have many other trails besides the AT, though I'm not familiar with them. Others can chime in with specifics for you.

Don't overlook Michigan: there's backpacking at Pictured Rocks along the shore of Lake Superior, and there's Isle Royale NP in Lake Superior - a spectacular place I've been to twice. I've also seen articles about the North Country Trail, Ice Age Trail, and Shore-to-Shore trail, but haven't visited them myself.

As far as other people giving you "the look" - give it right back. As far as I'm concerned, if they're physically able and don't try, they're the ones who are nuts. But then, one of my retirement goals is to become a curmudgeon...