Extending ruby in C - how to specify default argument values to function?

I'm trying to write a C extension to ruby that'll generate a class. I'm looking on how to define some default arguments to a class. For example, if I have this class decleration in ruby:

class MyClass
  def initialize(name, age=10)
    @name = name
    @age  = age
  end
end

You can initialize it with mc = MyClass.new("blah"), and the age parameter will be set internally. How do I do this in C? So far I got this, but this forces entering the other argument:

require "ruby.h"

static VALUE my_init(VALUE self, VALUE name, VALUE age)
{
    rb_iv_set(self, "@name", name);
    rb_iv_set(self, "@age", age);

    return self;
}

VALUE cMyClass;

void Init_MyClass() 
{
    // create a ruby class instance
    cMyClass = rb_define_class("MyClass", rb_cObject);

    // connect the instance methods to the object
    rb_define_method(cMyClass, "initialize", my_init, 2);
}

I thought about checking the value of age against Qnil or using if ( TYPE(age) == T_UNDEF ), but I just get segfaults from there. Reading through README.EXT leads me to believe I can accomplish this through rb_define_method using the value of argc, but this wasn't too clear. Any ideas? Thanks.