function validity(obj)
{
	var age=document.registerform.age.value;
	if((obj=="M")&&(age<21))
	{
		alert("You should have a minimum of 21 years to register with us");
		document.registerform.gender[0].checked=false;		
	}
	if((obj=="F")&&(age<18))
	{
		alert("You should have a minimum of 18 years to register with us");
		document.registerform.gender[1].checked=false;
	}
}