According to the website the Wengen - Mannlichen Cable car will begin operations May 17th but in reading various online comments about the weather it seems that historically there has still been a lot of snow on the trail to Kleine Scheidegg in late May.
I have read that it has been a mild winter in Europe so maybe this year it will be clear enough for an (ahem) older amateur that has been doing 5 miles a night on the treadmill to practice to make it to Kleine Scheidegg :-)