With date of birth in A1 and the date in B1; try the below
=DATEDIF(A1,B1,"y")&" years "&DATEDIF(A1,B1,"ym")&" months "
'as of today
=DATEDIF(A1,TODAY(),"y")&" years "&DATEDIF(A1,TODAY(),"ym")&" months "
For details around calculating age refer
http://www.cpearson.com/excel/datedif.aspx
'Another option. Please note the difference of the below funciton when
compared with DATEDIF
=--TEXT(TODAY()-A1,"yy")
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