Metabolism does slow down with age, mostly. The thing is that muscles expend more energy than fat. That means that if you can manage to at least not have muscle atrophy, and preferably add some lean muscle, you will be able to keep your metabolism higher. Our muscles DO atrophy with age and often an increasingly sedentary lifestyle though, so you really, really need to do some kind of resistance training.
It is actually worse to go on a diet and do tons of cardio. Our bodies are smart. Since muscles expend more energy, in a caloric deficit your body will much rather break down muscle for energy than lose the fat - unless you show your body that you NEED that muscle. A diet and cardio does just that; you WILL lose weight, but a fair bit of that weight will be muscle mass. Then when you go off that diet, which is typically the case, you have a slower metabolism due to less muscle mass, and you end up putting on the weight again, and a little extra too in many cases. That's why resistance training is vital. If you regularly challenge your muscles, teaching your body that it needs its muscles, then you will lose fat rather than weight, which is most likely the goal for most people wanting to lean out a bit.
Calories in versus calories out is still the best way to do it IMO. Put yourself in a slight caloric deficit by eating a well balanced diet that you enjoy, do some form of resistance training about 3 times a week and you will be golden. Do a bit of cardio for your heart and to expend a few calories if you want, but it isn't really necessary for fat loss. Just means you can eat a bit more food. And it is good for your heart of course!
