It depends on what information do you have and how accurate do you want?
For example:
For cell A1, type =TODAY()
for cell A2, type =12/25/1970
for cell A3, type = A1-A2
The result gives 5/16/35, that means 35 years, 5 months and 16 days old. BUT remember to format the cell A3 in the form of DATE (12/25/98 instead of 12/25/1998). Otherwise, it would shows a wrong data.