>Am I right to go back to the person's birthday knowing his age today?

>=MESSAGEBOX(GOMONTH(GOMONTH(DATE(),-18*12),-6)- 22)

You can't go back using a different calculation and expect to always get the same result. Consider someone born on Feb. 29, 2004 (a leap year). How old is that person on Feb. 28, 2005? I think most people would say the person is one year old exactly, which is what my age calculator says, too. Now, how old is that person one day later, on Mar. 01, 2005? Again, I think most people would agree the person is one year and one day old, which is what my age calculator also says.

If you work backwards from those results using the GOMONTH formula you suggest, however, then you will end up with Feb. 28, 2004 in the first case and Feb. 29, 2004 in the second case. Try the code below and see for yourself:

lox = NEWOBJECT( "AgeCalculator", "age.prg")
CLEAR
ldDate1 = {^2004-02-29}
ldDate2 = {^2005-02-28}
lox.CalculateAge( ldDate1, ldDate2)
?DTOC( ldDate1) + " to " + DTOC( ldDate2) + " = " + ;
   TRANSFORM( lox.nYears) + " years " + ;
   TRANSFORM( lox.nMonths) + " months " + ;
   TRANSFORM( lox.nDays) + " days"
?"Reverse calculation = " + DTOC( GOMONTH(GOMONTH(ldDate2, -lox.nYears* 12) , -lox.nMonths) - lox.NDAYS)
* Result:
* 02/29/2004 to 02/28/2005 = 1 years 0 months 0 days
* Reverse calculation = 02/28/2004

* and 

ldDate1 = {^2004-02-29}
ldDate2 = {^2005-03-01}
lox.CalculateAge( ldDate1, ldDate2)
?DTOC( ldDate1) + " to " + DTOC( ldDate2) + " = " + ;
   TRANSFORM( lox.nYears) + " years " + ;
   TRANSFORM( lox.nMonths) + " months " + ;
   TRANSFORM( lox.nDays) + " days"
?"Reverse calculation = " + DTOC( GOMONTH(GOMONTH(ldDate2, -lox.nYears* 12) , -lox.nMonths) - lox.NDAYS)
* Result:
* 02/29/2004 to 03/01/2005 = 1 years 0 months 1 days
* Reverse calculation = 02/29/2004

To go backwards and always arrive at the same starting point would require a more precise calculation than GOMONTH() alone is going to give you.

Whenever you work with date arithmetic you have make certain decisions about what constitutes a "month" and even a "year". The decisions I made for my age calculator class are documented in the readme file.