The following was written by a native speaker whose command of English is quite solid:
1763 Romney entered the Death of General Wolfe for a prize and won 25 guineas and sold the painting that year for a further 25 guineas. His fame began to spread.
He moved to a larger studio at Charing Cross and shortly after to Covent Garden where many artists lived.
Tools are just simplified representations, and can't in my view substitute for real brains evaluating real utterances.
I think the sentence is far from beautiful, but it's natural enough to be considered normal by a native speaker of AmE.
To me, it's clear "91 years" represents a duration, a length of time, a span, whereas "91 years old" is a single point on that continuum. Using "at" to locate the deceased on that continuum is just fine.
"He fell at the 91 meter mark" is a sentence one might say of an unfortunate sprinter.
"They examined the high water point."
So I think the word "old" can be situated and explained just fine.