WINDOWS OF DEVELOPMENT
Some milestones of normal growthhave what I call wide "windows of development"--a long range of time over which they occur.  If your baby reaches the milestone at any point within the window, don't worry.  He's doing fine.  For example:

Some babies walk well before a year, while others don't toddle until 18 months.
Some toddlers are ready to learn to go potty by 2 years, but other childfren (particularly boys) may not be ready for another 2 years. 

Some babies say their first words by 9 months.  Others will cooperate with brief requests like "Wave bye-bye' but cannot use clear, expressive language until nearly age 2.  Yes, that child might love to share picture books with you each day and be able to point out animals in the book. 

However, some developmental windows are much narrower.  You and your child's teacher need to be alert for the absence of a given skill a few months after the "norm" age is reached.  There are things you can do to help.  For example, if at one and a half months a baby still uses his fist instead of a pincer grasp, you should step in and provide bits of food such as grated apple for more practice with finger dexterity.  If your baby has trouble with walking at 18 months, be sure to provide safe and firm surfaces for practice and let him use his bare toes to grip the floor. 

If you have concerns, don't hesitate to ask your baby's teacher or pediatrician.  They might share good ideas for gently helping baby move toward the developmental milestones you are waiting for.