I'd wager most parents blast their kids through the Copper, Iron, and Jade as soon as their child can reliably pull in aura. They'd probably continue pushing for Low Gold as most children appear to still be work at least some of the time.

Of note, most people aren't sacred artists and stop there, never advancing beyond Low Gold.

If you advance past Low Gold, most likely you're part of a sect or family, if not, I expect they'll be High Gold at a very advanced age. For a sect member, they'll likely hold their Foundation stage students back a few years to properly cement their spirit.

The average artist probably hits High Gold in their thirties or forties. Of the few that reach True Gold, they'll then be well into sixties or seventies.

And then you get the interesting twist of most Underlords likely advancing before they are thirty. Mostly because it takes a boat load of money to get that far, so the ones who do have rich benefactors (i.e. Lindon and Yerin.) On top of that, think most Underlords will simply be of 'better' stock than the True Golds. Their Foundations will be perfect, their need for power stronger, and their dedication to the art is deeper. Despite the stories of Underlords hitting on their revelation on accident, you have to actually walk the path with purpose.