Youths going up from 'ok'

https://www.virtualmanager.com/players/17464360-taajuk-alataq

This player is training well above what i would consider an 'ok' player how likely is it that he or any youth player goes up from 'ok' to lets say reasonable/promising its happened probably twice in the decade or so i've played this game

Not as likely as this player will be on sale shortly haha

I did read potential can't be measured fully without ten trains
When you sell.after five, people are basically buying a lottery ticket on the potential potential
If that makes sense

My understanding (although on this occasion I can't point you to any specific information) is that "potential" is given at the birth of our new players and will not change much if at all over their lifetime.
What can and does vary is the level of their potential indicated by your Scout(s) - due to their own fallibilities.

https://www.virtualmanager.com/blog/1075-information-about-scouts