All fans have learned is that he was a Jedi youngling in the temple on Coruscant and was trained by many Masters over his years there.

Source: https://screenrant.com/every-jedi-alive-during-star-wars-outlaws/

Either way, Dooku casually encouraging a youngling to walk a dark path clearly shows where his future allegiances will lie.

Source: https://screenrant.com/star-wars-dooku-origin-retcon-wookiee-apprentice/

Instead of the fun and inventive youngling last seen in season 5, the Wookie was forced to mature quickly and adapt to the ways of his people to avoid a life on the run.

Source: https://screenrant.com/star-wars-gungi-order-66-survivor-tragic-future/

No, it is Ahmed Best's own Kelleran Beq who saves the little youngling from being killed by clones, wearing a very cool engraved robe and wielding two lightsabers like a badass (one of very few Force users to do so).

Source: https://www.slashfilm.com/1235927/the-mandalorian-gives-ahmed-best-the-redemption-arc-he-deserves/

The construction of Windu's most important tool proved to be a major moment for the youngling.

Source: https://www.cbr.com/mace-windu-failed-making-lightsaber/