A quote from The Guardian:
For it is mercilessly clear that when Porter died last year at the age of 55, he was at the height of his powers.
Should the definite article always be used when a single cardinal number is used in the expression "at the/an age of.." ? Or "AN" is also acceptable if the sentence does not refer to a definite person, like, say "students tend to graduate at an age of 22"?
I find numerous examples of "at an age of" and it's hard to sense the pattern of usage.