As already mentioned, health, experience, and even attitude matter. For me, the end of my travel days will come early, chronologically. I have two standards, one that may be flexible, the other that I don't think will be.
First, I must be able to carry my own luggage - one bag on one shoulder, with both hands free. When I can't do that, I'm going to be thinking hard about continuing. (Then again, I once thought age 40 was ancient, so maybe I will make a concession.)
Second, I have strong preferences about where I travel on scarce funds. I've spent plenty of time in Eastern, Central and Western Europe; Asia including the subcontinent; Central and South America; the Caribbean; most of the Middle East; Oceania; I'm not interested in going back. I have no interest in Africa-for-Tourists. So, what's left is tougher travel. (I know this because I've already spent much of the past three decades in non-tourist Africa.) Exposure to nasty disease, absence of good medical facilities, lots of operational hazards... I will soon be too old for all of that and travel will simply lose its appeal. That'll be sad.