Interesting article, but I think there's a huge hole in the information they are providing that may tell a different theory as to "why". They did touch upon the fact that it's really difficult to get accurate data on how many people are actually entering the backcountry. But I think this information is vital to telling the story of why the average fatality age is increasing. If there are a growing number of 30-39 year olds, but a less drastic climb in 20-29 year olds, then the numbers might even out "per capita".
My husband and I always joke that the natural progression for professional snowboarders is to compete in halfpipe or slopestyle competitions until you reach about 25, have a few "awkward" years, then progress to big mountain terrain in your 30s (Travis Rice, Danny Davis, Elena Hight, Nick Russell...just to name a few). Skiing is a bit different because racing is so big. Racers will tend to enjoy groomers, speed and don't mind the icy slopes (with the right tools)...there is less inclination to get out into the backcountry and jump into the freeriding atmosphere. While racing is alive and well, there are a lot more young people getting into skiing halfpipe and slopestyle...this translates to later years moving into freeride terrain, on a similar path snowboarders tend to take.
This theory doesn't only translate to professionals, but trickles down to recreational skiers and snowboarders. Now, if the data were to show that the number of older people in backcountry terrain is not growing faster than youngins...you can consider this post debunked and just a ramble.