A common reason people begin new fitness/health routines is because they begin to feel the effects of aging, whether that be weight gain, achy joints, or experienced weakness. Many people fear aging for these reasons but it doesn't have to be a fast slide into the prison you imagine it to be. A good preventative medicine plan includes strength, mobility, flexibility, and balance training. Sometimes it may seem counterintuitive to think of regular resistance training as a method of reducing discomfort, particularly because it may not seem like a comfortable activity while working out.
This is where a good coach comes in, so you don't have to design your own balanced, properly weighted program; your coach should be someone you enjoy working with, someone who inspires you and pushes you beyond what you thought was possible. I have never been stronger, felt better, and been pushed harder than when I was trained. We all have blind spots and a good coach illuminates those areas for us.
Even if you aren't ready to begin a full program, begin stretching yourself in your workouts. If you aren't accustomed to doing any exercise, that's ok; begin small by taking walks in the park or around the block. Start slowly increasing your distance and pace. As you feel more comfortable with these strides, begin incorporating some body weight training, such as squats and wall push-ups.
A common misconception about exercise is that you have to commit to a program that is going to kill you, and that simply isn't true and in fact will lead to greater rates of burnout. You are much more likely to stick with and enjoy an activity that you have slowly acclimated to.
Peak bone mass happens in our twenties and slowly declines as we age. Strength training helps slow this process, as weight bearing activity increases bone density. Studies have indicated that aerobic activity has a positive impact on those prone to or who have Alzheimer's disease. The best bet is to incorporate both aerobic and strength training into your lifestyle at a pace that works well for you. The payoff is priceless in the long run.