The easy question is when to strength train! Anytime that is convenient for you is the best time to strength train. Do it 2-3 times a week to build muscle and neural-muscular connections to help you stay on your feet with better balance and coordination, on non-consecutive days. At the very least, be out of your chair & on your feet for a minimum of 4 hours a day. If you have a desk job, stand up often and maybe even put that computer at standing level. I always want to get a plug in for doing at least 150 minutes of cardiovascular exercise per week with a good portion of it being weight-bearing exercise, meaning on your feet! If swimming is your favorite cardiovascular exercise, then make doubly sure that you strength train for the nice bone-building pull that it puts on bones.
What exercises to do is a bit more complex, in terms of strength training.
Take care when looking on the Internet for strength training for women, you will find many exercises which may be inappropriate for women in the peri- and post-menopausal years. Today, I saw one site that suggested doing pull-ups, push-up, lunges, and throwing medicine balls (large, weighted balls) . You need to start with much easier exercises than those! Push-ups and pull-ups aren't good to do if you have shoulder issues or a high body mass index, throwing medicine balls would be dangerous with low bone density & balance problems, and lunges can be fine only if your knees are fine. I'd recommend easing into women's strength training slowly, first focusing on form and breathing with easy exercises, then slowly adding challenging weights and moves. It is time well spent and will reduce your risk of injury.
Deciding which strengthening exercises are the best for YOU depends upon a number of factors: your fitness level, current health, injuries that you may have to work around, whether or not you have low bone density or arthritis, and your age, to name a few. What works for a 20 year old could injure a woman past 50, so it’s good to make sure that you begin at an age-appropriate level, tailored to your specific needs. University research shows that bone-strengthening exercises need to be site specific, targeting the muscles around the bones that tend to get the weakest from osteoporosis.
You will love what strength training can do for you! All of your other weight bearing activities will be easier when you are strong, such as dance, brisk walking, and biking. It is incredibly rewarding to feel stronger within a few weeks of beginning to strength train and within a year, to be stronger than you have been in decades. It’s nice to know that you are slowing bone loss AND reducing your risk of fracture because as you get stronger, balance and coordination also improve. You can then catch yourself better if you start to fall and have the strength to stay upright. If people simply don’t fall down, their risk of fracture is greatly reduced.