I think training not optimally, or your standards getting higher.
there are some biological processes that happen, slowing reaction time comes to mind. But I think people blaming aging is a cope. The worsening of the body is so slow, like even F1 drivers make it into their early thirties. So in my opinion 17-21 is almost definitely something else than pure aging, whether it be less physically fit, playing less, less healthy overall, drinking, etc