I will weigh in here, but my skiing is limited to resort steeps, mainly Mammoth now, and I never skied park much so I am not any help there. I spent half a decade in Tahoe at Squaw after college, I left a bit more than a decade ago. I skied 120 days a year then. I then left to go to grad school, and now I am a yuppie living in socal. I ski Mammoth 2-3 weekends a month, which is about 30+ days a year. I would say I am just as good at skiing the mountain as I was 7-8 years ago. Obviously I can't ski the way I could in Tahoe, but I ski far less. I have not noticed being unable to ski steeps I could 7-8 years ago, so I think I am just as good. For steep skiing I think you can maintain it into your 40's as a weekend warrior. As a local, I think you can maintain your skill, but you probably won't huck as big as you did when you are younger. Injuries and bruising yourself, while not as bad as in the park, still happens in the backcountry/big mountain setting, and so the body just does not want to do it as much. The older guys I knew in Tahoe were definitely that way. Still really good,could still tear apart steep terrain, but not quite as aggressive as they were when they were younger. As far as park goes, you definitely have more jarring landings, and you get stiffer as you get older, so I think park skiers have to cut back earlier than steep skiers. It helps to train in the off-season, if you want to last a while you had better do that.
I would not look down on becoming a powderhound though. I don't know any powderhounds regret leaving park to go to that kind of skiing. Keep that in mind. For me, powder skiing is the ultimate in skiing. It might not be for everyone, but I don't know anyone who skis pow and wishes they were skiing park instead. Even most park skiers that I knew in Tahoe lived for pow, they skied park when there was no powder. You never saw them in the park on a powder day. I'm not putting down park skiing, and I really don't want anyone to infer that from my post, just understand that there is a whole other realm of skiing out there that lots of people live for. Good luck. I do hope you are able to keep progressing for a while, I'm just saying that you might find you prefer pow when you go in that direction.