Most things are going to get worse for climbing as you get older at that sort of age, especially muscularity, raw power, injury resistance, and bravery / foolhardiness (also worth noting that if you plan on putting on any extra weight, as well as the power-to-weight detriment, there's the secret anti-bonus that it's bloody terrible for the effects on skin / finger pulp, thus being able to tolerate tiny holds gets worse too).
A few things can get better - technique, tactics, wisdom and cunning, and any untapped areas that you haven't previously tried to train (this being the illusion of progress as you get older) / tackle - as you say, stretching, for example.
The general concept of "get the burly stuff done whilst your body can still tolerate it, then bimble up slabs when you're a shuffling bag of injuries / weakness" does seem sensible, but taking into account the above, there's more nuance than that. As Barrows mentions, tweaky stuff is best done sooner rather than later (not the same as burly stuff), and other injurious problems could include anything wildly dynamic, drop knees, heel hooks, etc etc, again get them out of the way while you're more resilient (or just avoid because it could fuck things up for quite a while as you get older). Conversely, there could be other styles that are worth saving in addition to slabs - anything that's particularly complex, sequency, and knacky where you can use your wise old brain further down the line, or anything that requires a lot of nouse to get conditions and sending tactics right (which the yoof of today with all their damp grit climbing and resting 20 seconds between attempts usually lack).
And of course there's generally a lot of sense to saving easier problems until you're old enough that you can't do hard ones and the easy ones are then hard. And taking advantage of the massive amount of bouldering development and documentation that gives us vastly more options these days - one advantage to this current ridiculous bouldering fad.
