"When Nicholas staged his Second Exodus[,] the Osis brothers, now free of the camp, joined him and the nascent Clans. Simon was killed by a smoke jaguar while the brothers were on a hunting expedition and his brother’s death spurred Franklin to new heights, earning him command of Clan Smoke Jaguar and a key role in Operation KLONDIKE."
From Franklin Osis profile on HOK. He'd be 29 the year the Clans were formed.
It is not entirely clear what kind of career he had before this, but it is possible he had received MechWarrior training beforehand at some point, his parents were tank commanders so it is plausible he got into Exodus fleet military. I'd assume they would have been training new people even en-route and afterwards, despite large demobilization initiated by Kerensky. A military culture knows that passing skills on to next generation of soldiers is important.
It is also possible that Franklin Osis began his career as infantry, and received 'MechWarrior training later. While BattleMechs were the most valued thing by Nicholas Kerensky, there were warriors in the Clans and Khans who weren't MechWarriors. Considering that the profile notes his brother's death "spurring him to new heights", it is possible he trained as MechWarrior at that point, and trained quickly and effectively. Such thing would certainly impress Nicky.
Side note:
Interestingly and weirdly, there are two Osis bloodnames in Smoke Jaguars, with Franklin's being a MechWarrior name and the other being an Elemental name, possibly. Considering Franklin Osis past as a gangster and occasional hunter, him being infantry and progenitor of the Elemental line would've made sense but this is perhaps a coincidence.
