Damn me ...

[Antagonist]

There could be some reason why ALL Chronomancers might only have 100 years or so to play with.

Either a flavor 'deal', exchange of Chronomancer powers for a century, then they get 'claimed' by whoever they made the deal with. Perhaps a window in time that Chronomancer powers is possible before it is removed from the world, and you only play in that window. Or else maybe an event in 100 years time, like a huge war that would wipe out most of civilization, one that the protagonist is fated not to survive. Speeding up time just brings closer that fate, even if the event is still where it originally is supposed to be.

Wouldn't help much to explain the age advantages, aging an elf 100 years is unlikely to bring as much wisdom as aging a shorter-lived creature, unless you consider that 100 years as an adventurer would bring far more wisdom than 100 years as a civilian.