It all really depends on the snowfall this winter. I was there in April 2002, which I believe was a minimal snowfall year. I was in Breckenridge from about April 12 thru 20th.
Beaver Creek had already closed. The weekend of April 13-14 was the last weekend for Vail and I believe Copper Mountain. Keystone and Breckenridge were both open thru the weekend of April 20-21. And Arapahoe Basin is usually open into June/July. Remember ski conditions will vary greatly. In the morning it can be icy. And as the day warms up, it can turn a little slushy. Then as it starts to get colder, it can get icy again. And, as runs deterioate they will be closed. I believe Breckenridge was maybe 40% open when I was there. But with a good snowfall year, they may be open later and have better skiing.