It usually starts small. You get out of bed and your knees need a minute before they will cooperate, or the first few stairs have you reaching for the rail a little sooner than you used to. Push up off the floor after playing with the kids (or grandkids), and your hips and shoulders will make sure you feel it for a while afterward.
None of it stops you, of course. You are still active and still getting out there, it just costs you more than it used to, with longer stretching before a walk and a proper warm up before the gym that you never used to need. The stiffness lingers too, hanging on well after you expected it to ease off, so you do what everyone does and grab a bottle of joint pills, take them for a few weeks, and wait for a change that never really comes.