I think part of your confusion is because you have not compared 'like-for-like'.
The original sentence you quoted has an adjective: "fourteen-year-old"...
- but in your example, you change this to an adverbial clause of time: "...when I was 14 years old."
This makes a difference in the use of the past (or present) perfect tense.
So the original sentence is fine,
but you wouldn't say it like this:
* She'd been the fastest girl in the country at breakstroke when she was fourteen years old.* (you would say "...was... was...")
Equally, you could switch this sentence to the 'present perfect':
e.g. (in a conversation...)
a) I've heard of athletes who have achieved amazing things at a young age!
b) Well, yes. Look at me!... I've been the fastest 14-year old breaststroke swimmer in the country. (= One of my achievements in the course of my life, so far)
but not:
* She's been the fastest girl in the country at breakstroke when she was fourteen years old.*