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.
Answer
“At the age of” is the idiomatic phrasing. It very strongly dominates “at an age of”.
Browsing through published occurrences of “at an age of”, I see mostly scientific and legal publications.
“At an age of” is also preferable to express an unspecified point in an age range, as in “plant growth stops at an age of between 100 and 500 years” or “Walking difficulties are often detected already at an age of 1–3 years”. Using “at the age of” would indicate that the statement applies to the whole range rather than one point within the range.
Attribution
Source : Link , Question Author : CowperKettle , Answer Author : Gilles ‘SO- stop being evil’
