I turned 53 last month. I don't have lots of aches and pains and don't see much change in my energy levels. I have seen an increase in my weight and my middle seems to be expanding a bit despite eating a very clean diet.
I run, do weights, kettlebells and lots of functional fitness exercises. I do jump rope, jump squats, BOSU jumps, broad jumps, bear crawls and many compound moves with weights. I use weights in various forms (med balls, kettlebells, sandbags, barbells and dumbbells) in all my workouts except when I run.
I workout 5 days a week, eat clean and I am putting on muscle, especially in my upper body. I have to work my legs harder as that is where I tend to put on weight (genetics). I do lots of weighted squats and walking lunges.
The poster who said that she wanted to be functional in old age is exactly what I am aiming for. What I am doing at 53 I what to do at 83 and hopefully 93 and beyond.
If I do get a bit achy, I foam roll and stretch. I don't care for yoga or stretching (in fact I hate it) but foam rolling helps with any aches I get from running.
Everyone is different and you should listen to your body in terms of how far you can push it. The goal should be health, strength and functionally fit.
BTW...my mother is 80 and has had many health issues including stage IV endometrial cancer at the age of 76. She now goes to a physical therapist that has her doing pilates. She has gained so much strength from doing that. Before she could barely lift a half gallon of milk, now she can lift heavier items and even has some muscle tone. It's never too late to start getting healthy and fit.