In reply to Duncan Bourne:
For me the key to keeping on is to avoid injury, for which the main culprit is the indoor wall. When outdoor climbing is what it's all about, why would you risk injury on an indoor wall which will then wreck your upcoming outdoor season (of which, looking on the pessimistic side, you may not have too many left)? I use them, but very carefully - I have absolutely nothing to prove other than my common sense. I don't want another summer like 2016 which was non-existent on the climbing front after a shoulder injury suffered walking to the crag.
Personally, I don't believe you can do that much to stave off the decline. What you have to do is accept the inevitable, enjoy what you can still do without making yourself miserable over the things you can't any more. Every day out, every route, is a bonus for me. My climbing purple patch (well, more of a delicate mauve) is a distant memory but there's plenty of fun to be had on easy stuff if you have the right temperament for it. Some people give up completely when the powers start to wain, but that's really sad. My father, who I feel was too ready unthinkingly to accept the generally accepted, always said to me "If a thing's worth doing it's worth doing well" (with the implication that if you can't do it well, give up), but I long ago settled for "If a thing's worth doing it's worth doing badly".
So my advice is go sport climbing in the sun in winter, don't bother to go out in bad weather (unless you positively enjoy it) but wait for a nice day (you're retired now, you're not restricted to weekends) find a like-minded and congenial partner and get out and do what you can as often as you can.