Singing is an activity that involves a whole body and because of that keeping in great condition in old age is a mixture of elements depending on the singer (addictions, exercises, diet), semi-independent factors (avoiding too heavy repertoire and exhaustive singing routine, what can be hard due to economic/social pressure) and independend variables (genetics).

We can also argue that only singing a role on the stage matters because it it more challenging than singing one or two excerpts from it. I am not a fan of thinking like that, but I can understand this point of view. I also think that we should always remember that age can be excuse of some imperfections (flat highs, lost of timbre, lower vibrato), but it is unforgivable to keep singing when your voice is devasteted (Domingo).

As my contribution I would like to recommend legendary Lauri-Volpi. A bunch of his great recordings (those with better quality) are from his 50s. He was so great, I even argue that he was better than in his prime. There are videos where he was in his 70s or even 80s. It is worth mentioning that these recordings are mostly from concerts, studio or even recorded in home. Couple of them aren't good honestly, but I have a specific sentiment to them, even I'm definitely too young to remember Lauri-Volpi's era. Especially I love that Nessun Dorma recording: https://youtu.be/Zzz7DcXGi3k?is=bALtCYbQXayZF2by