I'm going out there soon and have looked at this closely. Most will close by Apr 15, Breckenridge closes Apr 22. Believe only A-basin and Loveland open after that and both vary their closing date depending on snow pack. A-basin is known to make it well into June.
BTW, fresh snow out there today.
Just heard Vail open until Apr 22nd also. Loveland usually goes until mid-May
Here's some very definitive closing date info for CO:http://media-coloradoski.com/CSCFacts/OpeningAndClosingDates/
Silverton and Echo Park also go to May.