Dr. Dobson is good, although I disagree with spanking 18-month-old babies. That can wait awhile.
While they are little, LOVE, LOVE, LOVE them.
The sin nature shows out in a child very early; earlier than 18 months of age. No the child cannot understand his nature at that age, but you do not wait to begin until a child can understand his nature. Outward restraint and discipline is necessary until there is the internal control of character, or better, the Holy Spirit of God.
Eighteen months of age is not too early to begin the use of the rod (Proverbs). Twelve months is not too young, provided that it can be properly ascertained that you are not treating sickness, hunger or a wet/dirty diaper with the rod.