The following information is posted with permission from Bird Studies Canada.
One question many people have is: "How old are my loon chicks?" These images may help you sort it out.
The first image is of chicks that are 1 day old – what we call "downy young". The cuties are little puff balls that are just learning to swim and look more like a cork than a loon. When they try to dive they pop right back up.
The second image is of a chick that is approximately three weeks old – what we call "small young". These guys start to look a wee bit like an adult loon but are still quite fluffy and are at least one-third the size of their parents. They are a little more independent but will still crawl onto their mom or dad’s back.
The third image is of a chick that looks considerably larger, about two-thirds of the adult size, and a little less-well groomed – think teenager here (large young). These fellows are around six weeks old and are starting to molt into more adult-like plumage. At six weeks old, these chicks are likely to survive long enough to fledge (fly off the lake).
The final image shows what the young look like after molting. They look considerably more adult-like, but they have brown bodies and eyes. The back pattern is scalier than the squares you see on an adult loon.