I'm "only" 41, but in sports terms that is already old.
I have certainly had to modify my training approach over the last decade. Stuff that I could pull of in my 20s would get me hurt now.
That said, I still lift heavy. Or let's say heavy-ish, as I have more of an endurance background and by nature am not so much of a "strength" type (even though I managed to bulk to ~110kg over time). 120kg back squats for reps or 150kg deadlifts for triples are not
that impressive, given my height and weight.*
As you get older, your recovery slows down. This means, fatigue management becomes crucial. There are some benefits that only heavy weights and near-maximal loads can give you - bone density, tension strength and neural adaptations. So I keep doing the heavy stuff. But I have greatly reduced the volume and the frequency of heavy lifting. I only deadlift and train with barbells once a month or so. Because with a "heavy" session, I can totally blow my lights out and I'll "live" as a zombie for at least the next tree to four days, but more likely for an entire week. I can't afford that - neither professionally, nor in regards to missed other training stimulus.
What has worked for me so far is to increase training frequency to near-daily training with load undulation, i.e. one or two taxing days, at least one very light day and the rest of the week light-to-medium. This of course requires to cut back on intensity and to reduce the volume per training session (overall volume still ends up higher). I also stay well clear of failure. I find that if I go to failure, my recovery time goes up exponentially.
Most of my lifting nowadays involves kettlebells - because of ease of use (take up no space, I can train from home, do not need to travel to the gym for a workout, etc), and because of killing several birds with one stone (explosivity, strength endurance, coordination, and a bit of cardio all at the same time, so it's very time efficient).
Another point hat has already been brought up is injury prevention. I find myseld doing stuff that does not have a direct carryover to my sport, nor does it help build muscle, such as yoga/indian calisthenics and indian clubs - just because it helps me feel better. To preserve cartilage, joints need to be moved through their entire range of movement under load. Restricted range of movement will lead to cartilage degneration. It will also greatly increase your risk of injury once a movement is conducted in an unusual manner, i.e. something slips, or an accident happens. So "maintenance movements" and mobility (which is way more than stretching!) plays a far greater part in my training that I would have ever thought it would 20 years ago.
To conclude: I think that most important to training longevity is balance. Balance load and recovery, neglect neither strength nor cardio, balance in muscular development (a lot of people neglect overhead work, which will mess up shoulders sooner or later), incorporate variety, preserve (or build up) mobility. We "older" athletes no longer need to make fast gains (we no longer have to be in top shape on a specific date to make it through seat races, to get selected for a team, to win a scholarship etc). All we do is for our own enjoyment; if we set ourselves competetive goals, we have all the time in the world. I am dead set to win German Championships at least once. But I don't care if I do that with 50, 55 or 80. Heck, the highest probability of success would probably be at 90 something. If I can manage to still be exercising by then, the field of competitors will be greatly reduced. And if at that age I am the only one still showing up at the lake and getting into a boat unassisted...
*
But then, I do not need max strength in my sport - above a certain threshold, you'r good. A rowing stroke will be 1,4kN when accelerating a boat from a standing stop and ~0,9kN during a race. That is, if your olympic-level-fit and have olympic-team level skill that allow you to actually get the power into the water. I'm nowhere near that good. So once your past a 150kg deadlift or you can power clean 90kgs you're strong enough. Any additional max strength is "only" injury prevention and will translate indirectly to a very minor improvement in strength endurance. I mostly train for work capacity and strength endurance. A master's race is 1000m, a full distance is 2000m. The latter is about 240 rowing strokes at race pace. That is a lot of work...