Your title question reads as a somewhat more mechanical point than your closing one, but perhaps I’m just mincing words.
Why does US still require the President a fairly high age (35) in
order to be able to serve?
Possibly more daunting than the requirements needed to amend the constitution is the incentive to do so in the first place. From a standpoint of their job, members of congress are elected to represent the interests of the people of their state/district. From that perspective, it would be a poor use of the public servants’ resources to pursue a change few of their constituents want or care about. At the risk of stereotyping, as we age it is fairly common to view the inexperience of those younger than us as less suited to leadership than we like to think of our older and wiser selves. So with relatively little concern among the voting-eligible population and over 2/3 of that population being over 35, it seems most of the constituency is unlikely to want their representatives make this happen.
Then consider the composition of congress itself. The vast majority is well over 35. This implies you’d need 2/3 of a collection of people averaging well beyond 35 to vote for it even if it were gaining popularity among voters.
Why not lower this threshold?
I read this as more philosophically pondering whether there is reason for the limit’s existence. hszmv already spoke to the founding fathers’ reasoning. There are plenty of quotes attributed through centuries to famous elders maligning the youth of the day https://proto-knowledge.blogspot.com/2010/11/what-is-wrong-with-young-people-today.html and examples of youth discounting their elders (OK Boomer). Philosophical questions rarely have a single truth. Here’s a barometer. Think back to a 1/3 of your life ago. Do you feel you were better equipped to and likely to make better decisions then or now? There are other factors, but overall experience usually counts for something. Assuming we would like “the leader of the free world” to make good decisions, it’s not the worst limit in the world.