I was rock climbing in early May of last year after ACL reconstruction in January. I believe the primary concern is the possibility of hyperextension or lateral stress during a fall/impact, so I wore a knee brace through the summer. It was a bit of a pain but didn't affect my mediocre climbing. I also never fell in a way where the brace mattered. Climbing itself (on rock anyway) didn't put any particular stress on my knee; your movements are generally controlled. I WOULD be careful if you are doing snow/ice stuff and wearing crampons. I've heard lots of stories about soccer players re-injuring their repairs wearing cleats and doing stop & pivot motions. So I can imagine maybe there is an analogous situation downclimbing that headwall if you slip or mis-step. The only time I caused injury to my knee after the surgery last year was carrying a pack back down the trail from Ingalls basin (July?). My foot slipped on a rock and my knee hyperextended. My guess is that your best chance of injury is humping your pack back down from your base camp at Baker. You're tired, there's a lot of weight on your back and the trails down are hard on the knees anyway. So I'd be careful on the snow and with heavy packs. But for rock climbing I'd say go for it if you're feeling good.