I agree with what Pam has said. We would like to switch ours to timothy based pellets too, but they turn their noses up at them and would rather starve than eat them! I have two extremely picky eaters on my hands (they won't go near fruits or veggies either) so they are maintained on an alfalfa based diet. I can only assume that this is okay as ours are very small, active rabbits that have trouble keeping weight on, and both of the breeders we bought them from fed alfalfa pellets to all of their rabbits (young and old). I am still a bit confused on the debate of alfalfa versus timothy though. Maybe time will tell...
I think the age that you would switch to a timothy based pellet is around adulthood...at the point when the rabbit is done growing. This varies by breed. Smaller breeds reach adulthood faster (about six months) than larger breeds (maybe 1 year for larger breeds?)