I’m 35 and I feel like I’m just getting properly started.
"Categorical age", if you'll allow me to coin a term for this phenomenon (infancy, childhood, adolescence, middle age, ... - your conception may vary) is more or less a qualification of our personal relationship to time and mortality. So while we might have broad consensus that a 5 year old is a child and a 70 year old is a senior, it's not determined by numerical age. Just a near 1 correlation.
After all, we never know our numerical middle age except in retrospect.
If you believe mid 30s is middle-aged, then it becomes middle-aged - and unfortunately, for some that might be true.
Fortunately for me, this crisis led to a serious investment in my health and fitness, and though I obviously cannot predict my future, I'm far healthier now than anytime in my adult life. I fully agree with the sentiment to not waste a midlife crisis.