Spaying being quite a heavy operation (you are still cutting the rabbit's belly open to take out internal organs ^^') most vets prefer to wait until the rabbit is about grown-up. It's more a question of size than of age. Also considering hormones have a function in the growing process I personnally wouldn't get one of mine spayed befor that either. So for a small to moderately sized rabbit the ideal age for spaying is generally around 7 month old.