expression: at 25 years old

hi,

How would you account for the expression "at 25 years old" ? Is it correct to write this way?

I think the meaning is the same as
"...when I was 25 years old", but what confuses me is the expression itself.

"At 25" only would do, but having "years old"
after the preposition "at" is somewhat strange. I've seen a good many examples and don't think is incorrect.
Could you have a say on it plz?

Srdjee