-- birthday is input parameter

birthyear    = extract( YEAR from birthday );
current_year = extract( YEAR from current_date );
age          = current_year - birthyear;
age          = iif( extract( YEARDAY from current_date ) >= extract( YEARDAY from birthday ), age, age - 1 );

create function calculate_age(
    birthday date
  , otherday date
)returns
    integer
as
    declare variable birthyear integer;
    declare variable otheryear integer;
    declare variable age       integer;
begin

    birthyear = extract( year from birthday );
    otheryear = extract( year from otherday );
    age       = otheryear - birthyear;
    age       = iif( extract( yearday from otherday ) >= extract( yearday from birthday ), age, age - 1 );

    return age;
end