You probably don't need scientists to tell you that your metabolism slows with age. As we age, we are subject to muscle wasting. Since muscle burns more energy than fat, this means the metabolic load goes down and metabolism reflects that. That is, if you do nothing about your loss of muscle with age, it will take you longer to burn off a candy bar at age 60 than at 20. There are things you can do to keep your metabolism efficient such as exercise, eat balance, eat often, drink green tea and drink water.
I found some body toning tips and ways that you can keep as references. Start to do it today and and say no on losing muscle and ageing. Pick any 5 of them and do 20 times as one set. Do 4 sets daily and you will see the result in 1 week. You will find yourself more tone-up, away from tired mind and not easy gaining weight like last time.