Just do it. It becomes a habit. Then second nature. Unless you are disabled or have a medical condition preventing from doing even mild sports you should totally incorporate some kind of fitness activity in your life. The sooner the better.
I've been doing some kind of sport since atleast elementary school. I never stopped being athletic. Without testosterone or steroids my peak has passed
The exercise you do now is a form.of insurance for your lifestyle into your 70s+. It can mean the difference between independent activity and mobility, and not being able to do much.
Not working out contributes to testosterone decline. Also lack of vitamin D. No reason to give up now, not when old people still run marathons. Use it or lose it.