Hi, I think the mistake you made is that "She'd been" = She had been So she WAS/HAD BEEN the fastest 14-year old.

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.*