Yes it's called brumation. Every dragon is different and I don't think anyone knows for sure what triggers some dragons to brumate and some not to and when. Cooler temps, shorter days - who knows. Since all dragons respond differently, I don't think it's possible for a definitive answer. It is a bit early though, so I would contact a vet just to be safe. One of my adult females started brumating a couple of weeks ago, but since it was so early, I took her to a vet for a check-up. She is fine and I don't have to worry.
My beardies, however, don't just suddenly go from active, big eating little pigs one day to brumating the next. It is more of a gradual process. Mine eat less, bask at the hottest part of their tank less and spend more time lounging lazily in the cooler parts of the tank. Some go from that lazy stage to brumation and others stay in that lazy stage all winter. Still others, however, stay perfectly active all year long. It's a very individualistic thing.
There is lots of info available on brumation and I'm not an expert - I can just say what my beardies do. I'd definitely get your beardie to the vet though - better safe than sorry!
Good luck,
Lisa