Over the years SAS has slowly restricted the availability of the fares while, in my experience, making them less attractive in terms of cost. You can still find cheap fares, so they are still a great value...but only when you can find them. In terms of restrictions, the youth fares tend to be blacked out around school holidays well in advance. Availability does eventually open up, but you might need to be patient and keep checking back. It also might depend if you are looking for feeder flights. I have noted the reduction if feeder flights combined with the reduction in youth fare seats have forced us to be a bit more flexible in the dates of the bookings. For example, MMX-ARN-ORD is usually cheaper than CPH-ORD but the availability of the MMX-ARN flight is often then limit. Then again, we have been known to book the occasional MMX-ARN-CPH-ORD voucher run when it comes available, so YMMV.

Please note my comments are based on experience booking youth tickets Scandi-US and not the other way around. In terms of school holidays, I refer to Sportlov (specifically week 8 in Skåne), hostlov (last week of October/first week of November) and Sommarlov (Jun 15- Aug 15). We have always found availability in the end, but summer has proven to be tricky in recent years.