Código HTML:
<script type="text/javascript">
function jump(a,t){
if(a.value.length == a.getAttribute('maxlength')){
document.getElementById(t).focus();
}
}
</script>
<form name="formulario" id="formulario">
<input name="dia" onKeyUp="jump(this,'mes')" type="text" class="Estilo7" id="dia" size="2" maxlength="2" />
<input name="mes" onKeyUp="jump(this,'age')" type="text" class="Estilo7" id="mes" size="2" maxlength="2"/>
<input name="age" type="text" class="Estilo7" id="age" size="2" maxlength="4"/>
</form>