New theory. When Clark was a teenager, he could buy beer for every one because he looked like he was 35 to 40 if he really "hasn't changed" since High School. Why would his super powers make his age rapidly to his prime, and then STOP ageing. That's Progeria weird. However? What if Kryptonians are a short lived race like the Ocampans from Voyager? So his natural maturation rate is to age to his prime in 15 years, what looks like a human 35, and then die as a withered old man of natural causes at a Kryptonian 30. This is when his super power kick in at around 15. His maturation rate slows and stops. 45 for the rest of his interminably immortal life.