Hi Luis
=DATEDIF(A1,B1,"y")+(B1-DATE(YEAR(A1)+DATEDIF(A1,B1,"y"),MONTH(A1),DAY(A1)))/(DATE(YEAR(A1)+DATEDIF(A1,B1,"y")+1,MONTH(A1),DAY(A1))-DATE(YEAR(A1)+DATEDIF(A1,B1,"y"),MONTH(A1),DAY(A1)))
where A1 is the birthdate and B1 the date that you want to calculate the
birthdate at - if you want to use the current date you can either put =NOW()
in B1 or change B1 in the formula to NOW()