How to set a default value with Html.TextBoxFor?

Try this:

<%= Html.TextBoxFor(x => x.Age, new { @Value = "0"}) %>

note that @Value has a capital V

This should work for MVC3 & MVC4

 @Html.TextBoxFor(m => m.Age, new { @Value = "12" }) 

If you want it to be a hidden field

 @Html.TextBoxFor(m => m.Age, new { @Value = "12",@type="hidden" })