Each year you have a birthday that determines how old you are. But there is another way to determine the age of your body based on your fitness according to Men's Health.
"We also know that for the average guy — let’s call him “you” — lung health and mental speed peak around your mid-20s. Beginning at age 30, your muscle strength and size start decreasing by about 3 to 8 percent per decade, and cardiovascular endurance dies off by about 1 percent a year. By 40, you’re slower on your feet. Once you hit 50, your brain is shrinking and bones are softening. From 60 on, it’s Murphy’s Law: What can go wrong will go wrong, all aches and pains and doctor visits." The expected age of an average male is about 76 years old.
It is all based on science - DNA, free radicals, cells in your body, and how well your endocrine system works.
You can take a test HERE to find out how old you are. Could be good to know to find more efficient diet and workouts for your body.