Try change the newOption initialization and text assignment:
function loaddropdown()
{
var dropdown = document.getElementById("ddFAge");
for(var i=1;i<=100;i++)
{
var newOption=document.createElement("option");
newOption.innerHTML = i;
newOption.value = i;
dropdown.options.add(newOption);
}
}
BTW, I am not sure where from you are using the value of i here. Above code would iterate 100 times and add the same thing to dropdown 100 times. I doubt if that is what you are trying here. Do check.