Did you know that statistically our basal metabolic rate decreases by approximately 5% for every decade of life after 20? People who have a sedentary life have a significant loss of muscle mass as they age.
Weight training and other resistance exercise will increase your muscle mass, which raises your metabolic rate and burns fat.
Diet alone won't cut it. If you are only using a diet plan to get lean, you are more likely to regain your bodyfat when you "go off" the diet. According to the National Weight Control Registry, 90% of successful dieters use exercise to maintain weight loss.
I'll be posting a quick bodyweight circuit training workout video on here in the near future.