When should castration be done?

Question
Our vet advised us to wait until our male chocolate lab is 18 months old before we castrate him.  By waiting that long, she said the dog's head would reach the proper size for his body.  Do other experts agree with this statement?

Answer
For years everybody said 6 months with it slipping toward 4 months.  I am still trying to verify what I found recently at http://www.caninesports.com/SpayNeuter.html  The link is not Google garbage, but from a site I find reliable.  If later spay/neuter becomes common, I hate to think of all the accidental breeding there will be.  

I nearly always tell people to go along with their vet unless he sounds like a complete kook. Yours may be ahead of the curve.