i´ve got this in the filemaker´s site to know the ages of my contacts (i´m learning to use LET function):

Let ( [ T = Get ( CurrentDate ) ; YT = Year ( T ) ] ;
YT - Year ( birthdate ) - ( T < Date ( Month ( birthdate ) ; Day ( birthdate ) ; YT ) ) )

what i did before was:

Let ( [ T = Get ( CurrentDate ) ; YT = Year ( T ) ] ;
If (
T < Date ( Month ( birthdate ) ; Day ( birthdate ) ; YT ) ; 
YT - Year ( birthdate ) - 1 ;
YT - Year ( birthdate )
)
)

both of them work perfectly

someone can help me to understand this: 
YT - Year ( birthdate ) - ( T < Date ( Month ( birthdate ) ; Day ( birthdate ) ; YT ) ) )

The expression in parentheses:

T < Date ( Month ( birthdate ) ; Day ( birthdate ) ; YT ) 

is a comparison; it returns 1 when true, 0 when false. So subtracting this from the difference between years accomplishes the same thing as your If() construct does. This has nothing to do with the Let() function.

The expression in parentheses:

T < Date ( Month ( birthdate ) ; Day ( birthdate ) ; YT ) 

is a comparison; it returns 1 when true, 0 when false. So subtracting this from the difference between years accomplishes the same thing as your If() construct does. This has nothing to do with the Let() function.

Thank you, so much!