I thought I would throw this up. I think its pretty obvious and I don't have any brilliant insight here. I have been skiing A- Basin in November for a few days every year in an effort to get some ski legs for the season. Conditions are usually a little spotty so I always take older rock skis and I always struggle for edge hold on the old skis. This year instead of old Super Shapes that have over 100 days on them I took a pair of TT 80s that have maybe 40 days on them. Wow huge difference (conditions were also excellent). While I was there I skied a pair of SS's that had plenty of days on them and had just gotten a new base grind. Compared to my TT80's they had limited edge hold.
Simple fact, skis lose their torsional rigidity over time and when this occurs the edge hold goes with it. I think this occurs somewhere between 50 and 80 days of skiing obviously somewhat dependent on where they have been skied and under what conditions and by whom (how large/heavy, how strong and how much were they bending the ski).
I have a pair of Head IM78's that just have no life in them and no edge hold. I have probably 100 days on them and they pretty much suck to ski on. They were my favorite skis for a few years and had great edge hold and a ton of pop.
Hey everyone needs an excuse to buy new skis. This is a very good one!