Some people just stop thinking at a certain point in life, I suspect.
Perhaps they think they've "got it made" in at least some way… have a job, got married, whatever… and they cease to actually think. Except in whatever limited domains require thinking.
Actually I am stating this wrong… maybe earlier in life, they had to think somewhat in order to achieve a level of competence or even expertise in some domain. But once they have that "mastered" they need think no more. About anything.
I strongly suspect "age related cognitive decline" is a myth. Why would humans live so long if the oldest aren't "useful" most especially via their intellects? An 80 year isn't good for battle or manual labor, but might be the key to survival if the mind has gotten better and better over those 80 years.
Rehash: I guess many in our time don't ever think at all, outside of some limited domain, and stop thinking even there when they think they've "got it". But it doesn't have to be that way.