It’s suggested in some books that young rabbits be given unlimited access to pellets but doing this can create issues later on.
As with people, good habits are often formed when young, and it is very important for your rabbit’s future health that they get into the habit of eating lots of hay. It’s much more difficult to introduce hay to an adult that has not grown up eating it. Pellets are extremely tasty and rabbits often prefer them to hay, so having unlimited pellets available can mean young rabbits eat little or no hay, a habit that can cause dental problems and make them more prone to digestive issues.
Pellets were designed for commercial breeders to make rabbits grow quickly, but in this situation little attention was given to the long term health or lifespan. Remember, wild rabbits grow up just fine on a diet exclusively plant material. Unlimited pellets are not necessary to rabbit’s development.
For these reasons, it’s a good idea to restrict pellets to a certain extent even in young rabbits, although they can have more than an adult.
I would read this article: http://www.therabbithouse.com/diet/diet-baby.asp