If you're still unsure of why you should strength train (it doesn't matter what age you are--better late than never!) Here's a helpful article that explains how the human body changes over time in regards to muscle loss and fat gain.
I also found a great posting on Reddit by Autiesocial that puts fitness and fat loss in terms of a car metaphor for easier understanding. Take the time to strength train--it's one of the best things you can do for your body. You only have one body--take good care of it.
Your body has a certain fuel efficiency rating that dictates how many "miles per gallon" of fuel (calories) you get. The higher the fuel efficiency, the more efficient your body is and the less fuel you need to get around. Bodies are designed to be very fuel efficient, so most of us have really high mpg. Because of this, we tend to put in a lot more fuel than we need, which our bodies thoughtfully store for us in the form of spare gas tanks (fat).
With me so far? Great.
Doing cardio is like driving really fast - you burn more fuel while you're doing it than your normal baseline mpg, but it doesn't do anything to change your fuel efficiency overall. So when you're not driving fast (which is most of the time) you go back to burning fuel very efficiently. Over time, if you really work at it, you can totally burn off those excess gas tanks with cardio, but now (since you weigh less) you need even less fuel to run at normal speeds. So it actually gets a lot easier to over-fuel and re-start that whole "extra fuel storage" problem all over again.
Muscles, on the other hand, are like switching from a 4-cyl to an 8-cyl engine (or more, for really beefed up muscle cars). You get more power and speed, of course. But at the same time, bigger engines will always burn more fuel, even when they're just idling (sitting on your ass or sleeping). So by building muscle, you essentially tank the hell out of your fuel efficiency. And since muscle is denser than fat, you can get a lot smaller while still weighing the same amount, meaning that not only do you have to fuel a "gas guzzling" engine, but you're still pulling a lot of weight around that has to be fueled as well. Only it's healthy weight - reinforced chassis and frame, enhanced stabilizers, pimped out exterior, etc - not spare gas tanks. So there's less need to cut calories, although you may still have to if you need to lose fat. Oh, and the whole thing works best if you use the high test stuff (i.e. eating clean).
Editing to add: This is a very general analogy meant to apply to "average Joe weight loss guy" and more specifically to fat loss (I realize cardio has other benefits, but that wasn't the original topic). Also, this analogy may not apply when cardio = HIIT (HIIT is it's own special universe of cardio with it's own laws) and breaks down completely when you are doing cardio at competitive levels (i.e. duh, Micheal Phelps doesn't have to watch what he eats.)
Photo courtesy of http://www.123rf.com/photo_5857614_a-female-fitness-model-flexing-her-bicep.html