The last few 100K on our tandem I've gotten cramps right at about 40 miles. One ride I cramped so bad at a stop that we fell over. I didn't have this severe of a problem last year despite being relatively new to cycling and in my 60's. I'm drinking, taking electrolytes, etc. Any grade at 9-10% or over I'm working very hard on the tandem and beating my quads up pretty good especially since we don't stand much.
It's hard to compare the effort on my single versus the tandem and I don't ride the single much, but about two weeks ago on the single I could really feel a difference in my leg stroke on the single versus the tandem. So I measured the crank to saddle on the single versus the tandem and found 79cm on the single versus 78cm on the tandem. Raised the tandem seat 1 cm and we took a 47 mile tandem ride yesterday which was trouble-free. It was a relatively flat ride, but I was pushing hard most of the way; the extra cm just seems to let me use my legs more efficiently.
Doesn't have much to do with being over 50 except us older bodies are not as forgiving as they once were.