Your select will look something like
"SELECT * FROM mytable WHERE birthdate <= $startdate AND birthdate >= $enddate"
You might also find the following function useful:
/**
*
* Display a form to enable you to select a date
*
* e.g. To show a start_date and end_date form:
* date_selector("start_", $start_date);
* date_selector("end_", $end_date );
*
*/
function date_selector($inName, $useDate=0)
{
//create array so we can name months
$monthName = array(1=> "Jan", "Feb", "Mar",
"Apr", "May", "Jun", "Jul", "Aug",
"Sep", "Oct", "Nov", "Dec"

;
//if date invalid or not supplied, use current time
if($useDate == 0) {
$useDate = Time();
}
/*
** make month selector
*/
print("<select name=" . $inName . "month>\n"

;
for($currentMonth = 1; $currentMonth <= 12; $currentMonth++) {
print("<option value=\""

;
print(intval($currentMonth));
print("\""

;
if(intval(date( "m", $useDate))==$currentMonth) {
print(" selected"

;
}
print(">" . $monthName[$currentMonth] . "\n"

;
}
print("</select>"

;
/*
** make day selector
*/
print("<select name=" . $inName . "day>\n"

;
for($currentDay=1; $currentDay <= 31; $currentDay++) {
print("<option value=\"$currentDay\""

;
if(intval(date( "d", $useDate))==$currentDay) {
print(" selected"

;
}
print(">$currentDay\n"

;
}
print("</select>"

;
/*
** make year selector
*/
print("<select name=" . $inName . "year>\n"

;
$startYear = date( "Y", $useDate);
for($currentYear = $startYear - 5; $currentYear <= $startYear; $currentYear++) {
print("<option value=\"$currentYear\""

;
if(date( "Y", $useDate)==$currentYear) {
print(" selected"

;
}
print(">$currentYear\n"

;
}
print("</select>"

;
}