I watched a video once of an adult that taught himself perfect pitch. It turned out the way he learnt it was a little different than how a child might learn it as he had to use reference points in his mind and then use relative pitch to calculate the actual note. This would take him quite a long time compared to someone who had learnt it young who could tell you the note straight away almost without thinking. I can't find the video now but there are many other similar videos on YouTube.