When do Sebastopol feathers start to look Sebastopolly? I've tried looking everywhere online and can't find anything about when the feathers start to curl.
9 to 10 weeks old they should have feathering # 1. The next Spring the feathering is as good as it probably will get, at 1 year. On the breast, ruffling is a good sign that the goose will be curly all over.
Here is a good picture of different feathering in Sebastopols. The white it the background is Zeena, who only gets a few long scapular feathers, but is extremely "fluffy when in full feather. The gray, her mate Zander, is a smooth breasted and their son Zen in the front, is a smooth breasted with better feathering. Zoey, their daughter in the next picture is curly breasted. Zucherro in the next picture, their other son, has developed some curl on the breast at 10 months old. Please note, colored Sebs are a work in progress and aren't always as curly as whites.
This years goslings won't look really poofy until this Fall-Winter.
I have found that their feathering will improve by the following Fall if they're genetically equipped to do so.
But be forwarned...they are in prime feather during the Fall but Winter and the breeding season takes its toll on their long streamers. By the end of the breeding season (late Spring) they look pretty crappy until they molt again and re-feather during the late summer through Fall.
Even Sebs with great feathering don't retain their long flowing feathers year round.