I've started noticing this since having kids. Not got a concrete answer but try some of the following:
* Drink as much water as you can and try and eat a mixture of freshly prepared food when you can.
* Work out and pick something you enjoy - currently I go kayaking, go to the gym and do yoga during the week and try to get out at the weekend into the countryside.
* Walk when you can, get outside and breathe fresh air
* Try to sleep (or at least be in bed) for around 8 hours a day
* Try to relax doing something not involving computers or tv each day
I was starting to wonder that my parents in their 70s seemed to have more energy than me but sleep deprivation can get you down after a while. Be positive and do what you can - it can be a downward spiral otherwise!