Method A: Convert birthdate to age with subtraction
Normally we figure out someone's age with current date subtracting the given birth date. So does it in Excel.
Step 1: Enter current date in a blank cell, such as 2012/6/18 in Cell B2.
Step 2: In another blank cell, enter the formula =(B2-A2)/365, and press the Enter key.

Then it shows the age in the cell. However, you may get a strange age such as 23.70411. But the following function will figure out a normal age:
=INT((B2-A2)/365)
Method B: Convert birthdate to age with DATEDIF function
Some would like to apply the DATEDIF function to calculate the age. Enter the formula =DATEDIF(A2,NOW(),"y") in a blank cell, it calculates the age immediately after pressing the Enter key.

Method C: Convert birthdate to age with ROUNDDOWN function
Another function to convert the birth date to age is =ROUNDDOWN(YEARFRAC(A2, TODAY(), 1), 0), which will figure out a standard age, such as 23.

Method D: Convert birthdate to exact age with DATEDIF function
Sometimes exact age is requires, and you may want to know how many years, months, and days from the birth date to current date. The following formula can help you figure out:
=DATEDIF(A13,TODAY(),"Y") & " Years, " & DATEDIF(A13,TODAY(),"YM") & " Months, " & DATEDIF(A13,TODAY(),"MD") & " Days"

See the screen shot above, this formula will show you precise result, such as 23 Years, 8 Months, and 14 Days.