As with all formulas that approximate the year length as 365.25 days, this
formula doesn't work when the 2nd date is close to or on the birthday.
I modified your formula to substitute a reference to cell A2 for TODAY(). Then
with the dates 2/27/2003 in A1 and 2/27/2004 in A2, your formula gives 0;11
instead of 1;00.
=ROUNDDOWN(((A2-A1)/365.25),0)&";"&ROUNDDOWN(MOD((A2-A1),365.25)/365.25*12,0)
With 2/27 as the month and day, the formula fails every 4 years, giving 4;11
instead of 5;0, 8;11 instead of 9;0, 12;11 instead of 13;0, etc.
If the person was born in a leap year, but after 2/29, and the 2nd date is the
birthday, the formula result is correct in leap years, i.e. ONLY 1 year out of
4.
To get the same answers as most people would derive based on the calendar, you
have to a formula like DATEDIF. See the other response in this thread.