Mike Robertson posted a great blog post on training an aging athlete - aimed at the athlete, not at the coach. It's a good guide to what to do when you're not as young as you once were.
I would add three ideas to the advice there.
Find what works for you, not for them. - As you get older, it's hard to do many things you once did with ease. Accumulated injuries, surgeries, etc. can catch up. Don't conform your training to the expectations of others, or the training prescriptions meant for young, able-bodied and still-growing athletes.
By all means expand your repertoire. But don't be afraid to ditch an exercise if it's just not working out for you. If barbell bench press hurts that often-injured shoulder, don't barbell bench press. Find another exercise that gets you the results you want. It's about getting the results, not about to the path you take to get there.
Lift for today, not for yesterday. - It can be hard to adjust to reduced strength, reduced total training time, and so on. If you lifted before and were stronger before, that doesn't necessarily mean you can lift the same weights for the same reps today. If you pulled 315 last year but haven't pulled recently, don't base everything on pulling 315 again.
Don't let the weights you trained before hold you back. You need to learn how to feel
Be realistic about it. You may be stronger than before. You may not be. But don't let your knowledge of what you did before set your weights for today's workouts.
It can be hard - 20s used to be a light warmup, and now they are a working weight. So be it. Again, it's results, not the path to get there. By all means aim to get stronger, and to set new PRs. But don't let your ego choose the weights, sets, reps, and frequency of workouts. It's now that matters when you lift, not what you did good or bad before.
Quality beats quantity. - Chronic injuries can come from repeated low quality movements. Incorporate mobility drills into your warmup and cooldown (if you do a cooldown). Emphasize perfect technique. Make sure the exercises you do are building you up, not beating you down with repetition after repetition of incorrect technique. Hire a coach if you need one, or just if you can. Aim for perfect adaptation not just adaptation.
All of this advice should apply at any age, but as you get older, it's more and more important to respect what your body is capable of, what you can do now to get better, and to make sure you do good work not just more work.