I've said it time and time again, but sometimes this topic creeps back up in conversation since there are still non-believers out there ;) It's true - strength training happens to be one of the closest things to a "fountain of youth" that we know of. I believe that this is such an important idea that it deserves its own blog post!
Yes... getting older is inevitable, but it has been found that doing resistance training can help you retain and build lean muscle, reduce fat, and maintain bone density for a way way longer time frame than if you didn't do resistance training. Not only will you look, feel and think better, but (proper) strength training can actually prevent and even reverse many of the injuries and symptoms of aging. It's never too late to start... or to make improvements to what you've got! Not to knock cardio entirely (there are still some benefits to it), but if you want to be efficient with your gym time, strength training has the upper hand as far as changing body composition and keeping you healthy. "How many days per week should I strength train?" you ask? It depends! It varies based on need and ability level. For most, I would say at least 2 days per week, and if your fitness level, recovery time and schedule allows, there's absolutely nothing wrong with 7 days a week!
This article was pretty decent on explaining the benefits of strength training, although I encourage you to read more on the topic yourself. The list goes on!
http://www.fitsolutionsblog.com/strength-training-will-keep-you-young/