I have read most of Ferber's book and I believe he suggests around 5 or 6 months to begin sleep training. You should also talk to your pediatrician as there may be other considerations other than age (e.g. weight, if your baby was a premie, etc.). I started a bit earlier out of desperation (around 3 months). My opinion is though if you are not ready to be consistent and deal with some crying, then don't waste your time. I would also suggest if you do try it, start over a weekend. That way if you are up at night, you have the day to recover.
I used Ferber's method and I am a huge fan. It is tough, but if you are consistent you will see results in only a few days (I started to see results in a day). My daughter is now 16 months old and is typically a good sleeper. Recently between colds and teething has not been sleeping great. So, once her teeth come through and she is feeling better, we will need to "re-Ferberize" her. His book talks about this too. I would suggest buying his book as I found it a good read. Good luck whatever you decide!!