In the past, when you turned 30 your were a nobody unless you had won the Nobel Prize in Physics. Thirty was getting very close to when you had to have a mid-life crisis.
At present, the Baby Boomer cohort has made several changes as they pass through. Life now begins after 40. Levis have a scouch more room.
In the future, you will have to be 70 to run for President of the United States. Franklin Roosevelt proved that you could be president for as long as you wanted. Harry S Truman proved that anybody could become President. Dwight David Eisenhower proved that the country didn't need a president until Barack Obama provided such a convincing counterexample.
At present, the United States Constitution is considered a living document. The 35 years in the Founder's document should be reinterpreted as a mature adult. Ronald Reagan was elected as a mature adult. Donald J. Trump qualifies on age. Senator Cruz should not be expected to be much more mature than Barack Obama.