What age difference is acceptable?

Some of you have probably heard of the "half-your-age-plus-7 rule/minus-7-from-your-age-and-double-it rule." For me, as a 24-year-old, this means that my girlfriend should be at least 19 or at most 34 (my girlfriend is 30).

What do you think of this rule? Do you have age limits for your dating partners? What is your opinion on large age gaps?