Enter date of birth in A1, the in A2 you may choose between:
=DATEDIF(A1,NOW(),"y")&"y,"&DATEDIF(A1,NOW(),"ym")&"m,"&DATE DIF(A1,NOW(),"md")&"d"
or
=DATEDIF(A1,TODAY(),"y")&" Years, "&DATEDIF(A1,TODAY(),"ym")&" Months and "&DATEDIF(A1,TODAY(),"md")&" Days"
You may replace "NOW" by "TODAY" or another date (or cell) and you may also remove the "days" bits.
MP