This forum is always joking about trying Alfie at 12.
But more seriously often suggesting, Jacob or Charlie at 12, Odogwu at 13, and probably some others I can't think of.
We've actually seen Bassett at 13, MLB and Fekitoa playing 12 or 13, Miller at 10 and 12, Oghre at 7, and many other unusual positions.
So many positions require comparable skills where a good player can move
Also, with younger players, they'll have often recently played other positions before breaking into the first team
And older players may have the intelligence and experience to adapt their skills to various positions too
Is playing Jacob at 15 as ridiculous as Oghre at 7?
Odogwu had some very good games at 13 that brought him onto the England radar, however, before that he was a winger that looked like he might have quick feet and some power, but he's now considered a winger / outside centre.
You only find that by trying players out in positions where their skills might translate....... But with so few dead rubber games these days it's incredibly difficult for a coach to try that.