|Drugs probably are the easiest solution ...
You could try all sorts of hard core training programs, but I reckon you don't need them to climb low 20s. I think the main answers to keeping up some sort of level with aging are climb lots, maintain a healthy weight, don't get injured (and hard core training programs are a great way to get injured) and pick routes that suite your style. There's world of difference between In lemon butter, Hidden secrets, Glory rodent, Tyrant's Grasp, Auto de Fe. If powerful moves or small holds were never your forte, they probably aren't going to become so now. So take every advantage you can get from earlier strengths. If you like slabs, cracks or techy faces, just stick to those. No shame in being a specialist! Work on technique, core strength, flexibility and endurance if you want to train, as these are the least likely things to injury yourself working on and make a huge difference to most people's climbing. Fingerboards, campusing, hard bouldering are all much higher risk, and honestly, unless you are trying to climb a lot harder than 22, you don't need that sort of power or finger strength.
In Lemon Butter is one of those routes I often have a difficult experience on - I struggle up the start and cruise the top or cruise the bottom and die of terror on the top ... There are many 22s at the Mt I would lead again before that one! You could try leading Crooked Mile to TR Hidden Secrets or Brick Dust to TR Squeakeasy. When you find something challenging enough that doesn't feel risky on pesky fingers, elbows and shoulders, do a few laps on it for training.