A true bubble rider sprite, I am told, is smaller than an acorn when fully mature. An entire classroom of children can fit inside one acorn, as long as there are plenty of windows. I assume that this is the reason we never see them, only the evidence that they were here.
The adult sprites look forward to retirement, just like we do. If you ever looked in your glass of soda, you can see that a fully grown sprite would not get very far on a bubble. Their memories are clear, though, and they love to tell stories about their days of high adventure. I’m not sure we can expect them to be entirely truthful, though, so I have a special face I make when I think they are pulling my leg.
By the time a bubble rider has outgrown her ability to ride a bubble, and then completed her task of sharing her memories, she is ready to become an ordinary fairy. This moment is measured by fitting an acorn’s cupula on her head like a hat. When it no longer fits, she has transformed.