DIET??

When you get older your metabolism slows down, which is why you will gain weight even if you haven't change your eating habits.

I don't think you should go on a "diet" if it means reducing food intake only and only until you reach a certain goal weight. However, you could go on a diet if it means a permanent lifestyle-change.

See your doctor first. Your physician might be able to shed some light on your weight gain.

Problems that we didn't experience earlier in life have a way of rearing their ugly heads when we least expect them to. Rather than assume overeating, age or lack of fitness as the problem, engage in meaningful dialogue with your medical professional to at least ensure that problems are not due to medical issues which might require prompt attention.

I agree with Kristen - lets find out what you're eating first. 

I've lost count of the number of clients who arrive in my clinic, announce that they eat a really healthy diet but.....and go on to describe their symptoms. Upon further questioning, it turns out that what they thought was a healthy diet actually wasn't

Also Jamie's suggestion is a good one, your doctor's clinic is a good place to start any investigation around your health.