It's nice that you start noticing it; we all age, whether you want it or not, and we deal with it, albeit each of us a bit differently. You will soon discover that sleep and food is really the only things you should be worried about, in that order. Of course, exercise when you can, but not for the muscles but for your sake of staying fit. You mentioned metabolism and that is true, it does change and fast and one needs to keep up with that. I did a personal switch from eating when they told me (hey, its lunch where are we going to eat), to "am I really hungry?" If yes, I eat. If not, I skip lunch, period. Super simple, eat when you hungry... Otherwise you will gain pounds like crazy and you will be wondering, "I didn't do anything differently." Yes, you didn't, your body did. You don't need as much food as before, the sooner one realize that the healthier one will be. Also, forget the numbers, you are X old and it does not matter, its just a number. It matters how you feel. Have fun getting old...lol We all going there....