I have a little girl hatched on the 16th of July who is broody right now- She has only recently starting laying, but has herself a lovely little nest in the corner of the night pens. While both ducks are still young- everyone has a first time- and some do fair better than others. I wouldnt say that they are too young to sit- size wise they are full grown, but maturity wise could be another thing all together. Who really knows if here is some age that the mothering insticnts of a duck kick in.. Just being broody for a start is a good sign, but some are better at sitting than raising the ducklings when they finally hatch.
Making sure they do eat enough during the 4 weeks is something I would watch for in younger ducks since they really can loose condition so fast when sitting.
Good luck with your girl- hope she does stay put for you .
