Thought I'd add my two cents worth in.

I believe there are VERY few people that have perfect pitch across all 12 notes and multiple octaves. Most people are stronger in certain ranges and pitches or even depending on what instrument is producing the pitch.

Typically it gets harder and harder, at least for me, the lower the pitches are. I have much more difficulty picking out pitches in low bass lines and they start to run together. In middle and upper registers there are many notes I can recognize immediately but some are more ellusive.

I also have difficulty when there is lots of distortion or effects on the notes. So some guitar lines I struggle to learn because my ear has more difficulty in isolating the pitch.

I personally believe everyone can improve their ear as long as there is not an actual physical defect. Just like most anything else some people have a natural talent (or perhaps an unnatural talent) and others have to work much harder for the same results.

Even though I don't have true perfect pitch I really messed myself up playing with my synth tuned down a half step for about 3 months. I was playing with a rock band whose singer benefited from the half step drop. I didn't even realize how messed up my ear had gotten until I picked up my guitar and everything just sounded so wrong. Never again will I play transposed for any length of time because it messes up your mind to play one chord and hear another. Eventually your brain just starts to accept those as the true chords.