At what age is it ok to let your baby cry himself to sleep, so he'll quit waking up at night?

Sleep hygeine: Dr.Ferber says 6 months in his book, "how to solve your child's sleep problems." which has been in print since the mid 1980"s. Certainly, beyond 9 months there is no physiologic need for babies to be fed in the deep night/morning hours. The longer one does this, it reinforces the habitual awakening and the ever more deeply entrenched"habit waking".Hard advice for many to follow.