Problem:
The birthdate field on the Contact object doesn’t show previous year and neither does it allow to switch back and forth the Years part easily.

Solution:
Below example will show the last 100 years.

1. Go to Setup -> App Setup -> Customize -> User Interface. Here make sure the ‘Show Custom Sidebar Components on All Pages’ is checked.
2. Go to Setup -> App Setup -> Home Page Layouts. Make sure all your Home Page Layouts have the Messages & Alerts component checked.
3. Go to Setup -> App Setup -> Home Page Components. Here, click edit for Messages & Alerts. In the textarea, copy and paste the javascript code below and save (it can just go below your normal Messages & Alerts, won’t show up on the actual page).

<script src="/js/dojo/0.4.1/dojo.js"></script>
<script src="/soap/ajax/11.1/connection.js" type="text/javascript"></script>
<script type="text/javascript">
dojo.require("dojo.collections.Store");
dojo.require("dojo.charting.Chart");
dojo.require('dojo.json');
var arYears = getYears();
function swapYears(){
	if(document.getElementById('calYearPicker') != null) {
		var select = document.getElementById('calYearPicker');
		var curValue = select.value;
		var parentx = select.parentNode;
		parentx.removeChild(select);
		select = document.createElement('select');
		select.size = 1;
		select.id = 'calYearPicker';
		select.name = 'calYearPicker';
		parentx.appendChild(select);
	}
	if(select != null) {
	for(x=0;x<100;x++) {
		select.options[x] = new Option(arYears[x], arYears[x], false, false);
	}
	}
}
function getYears() {
	sforce.sessionId = getCookie('sid');
	sforce.connection.sessionId=sforce.sessionId;
	var out = [];
	var currentTime = new Date()
	var year = currentTime.getFullYear()
	try {
		for(x=0;x<100;x++) {
			out[x] = x+year-99;
		}	

	} catch(error) {
		alert(error);
	}
	return out;
}
dojo.addOnLoad(swapYears);
</script>

Credits: Salesforce Community (http://boards.developerforce.com/t5/General-Development/Date-of-Birth-field-Calendar-years-don-t-go-back-before-this/td-p/120133)