There are at least three questions more important than "how old are you?" when it comes to soloing...or wilderness tripping in general.  In no particular order they are:

What kind of shape are you in mentally and physically?

How heavily are you packed?

How aggressive/easy is your chosen route?  Does it match the answers to the first 2 questions?

If you can get to the right answer on the first 3 questions then I think the age question becomes irrelevant.

A few years back Tim and I saw a soloist on the route between Joyce and Kash, which aint exactly for the faint of heart.  He appeared to be in his mid-70's and was completely unfazed by the rugged terrain of the portages.  He had his stuff in smaller packs that he was able to easily handle and he was in great shape.  Being retired, he was in no hurry, either.  Meanwhile Tim and I were staggering under our usual ginormous packs stuffed to the gills...with still more crap carabinered to the straps.  As usual, our travel days were super-aggressive in order to get the absolute maximum out of our 10-day vacation.  I made a mental note at that time that maybe we might want to change our style a bit as we age.  ;D

As I get older, I'm discovering wisdom makes an excellent substitute for brute force.