Realistically, in 3E, how long in a character's life do you think it takes to get up to level 13?
I mean, in just one or two battles, which could take place in a single afternoon for a character, it's easy enough to get from L1 to L2.
A hearty adventurer that experiences combat and other challenges on a fairly constant basis could get up that high in a very short time. While it may take years in real-life play time... for the PC in the campaign world, we're talkin' what... a year?
For example, the events of Return to the Temple of Elemental Evil are meant to take a character from level 4 to level 14. Playing once a week, that campaign takes about a year to play in RL... give or take. However, the in-game events can occur over the course of just a few months. If you tag on the time it took to get from level 1-4, it's still not that much.
So, I don't think that's an unrealistic metabolic rate for a weretiger.