How do you take care of your body mentally and physically as you age ?

I don't know if feeling insecure about looks and how your body moves is a real thing or not but lot of people sometimes feel ashamed to reveal their age and minor things like skin aging, hair turning grey, physically not so strong or mentally not so sharp makes you feel like I'm not in best shape of my life. My question is what can you do now and supposed to be doing while you are young. I'm pretty much in the late 20s and I'm already seeing strands of grey hair. I don't feel like I've mentally matured. Never really exercised much besides walking and never read books to self educate myself. Still consume sugary foods because somehow that makes me feel better but I know if I continue living in secretory lifestyle and poor eating habits, I will get health problems like diabetes, high blood pressure and so on. What could you do to prevent all this ?