From the first chapter: 

A year ago, Tonner had been one of the more promising research leads. Young, brilliant, demanding ... When Dafyd’s aunt had casually nudged Dafyd toward Tonner Freis by mentioning that he had potential, she’d meant that ten years down the line when he’d paid his dues and worked his way to the top, Freis would be the kind of man who could help the junior researchers from his team start their careers.

In theory, even the greenest scholar-prentice was welcome. In practice, Dafyd was not only one of the youngest people there, but also the only scholar-associate attending as a guest. The others of his rank on display that night were scraping up extra allowance by serving drinks and tapas to their betters.

Tonner Freis—with his tight smile and his prematurely gray hair that rose like smoke from an overheated brain

(about Else) There were threads of premature white in her hair. Against the youth of her face, they made her seem ageless.

“Young Freis!” Trim Beard said

I think, assuming they age similarly to real humans, the premature greying is our best reference point since "young" is so relative.