Show me anywhere that says a pirate overlord begins at a apecific level?
If you insist...
There is no mechanical power called "pirate overlord" that you need to activate. There is also no power associated with a pirate overlord. Skills, feats, and gear are all that is needed in this situation and all are provided for. The enemy ladder doesn't exist in all aspects of the game.
This is true, but let's look at some of the way we can represent a "pirate overlord" in a number of different D&D editions.
In AD&D, a pirate overlord sounds most analogous to a fighter lord, ie a 9th fighter whose has attracted their band of armed followers. Instead of a castle/hold, they've got a pirate ship, or perhaps a small fleet.
Now I suppose you don't
need to represent a pirate lord using a 9th level fighter. You can disregard AD&D's explicit connection between class levels and power/standing/followers. You could argue those rules are for players -- which is also true. But I'm fairly sure the most
common way to model a pirate lord is with an NPC w/a bunch of fighter levels (or another class).
Vanilla AD&D is interesting because it doesn't have a general level-based skill system. So technically, a 1st level NPC could be a master of all thing piratical (seamanship, navigation, bullying). A high CHR 1st level character is as able to influence & command people as a 20th level one. One could imagine a very smart * charismatic pirate lord who didn't rise to that position through martial skill alone (which would seem the most common route, right?).
AD&D 2e works pretty much the same as 1e.
Now 3e & 4e are quite different. Both have skills systems as core. This changes the equation. Low-level characters
cannot be an experts in their fields (barring cheap tricks like half-elf uber-diplomats). So in both 3e & 4e, lower-level characters have neither the combat skills nor the non-combat skills to justify the rank of "pirate lord". Using them to model "pirate lords" seems like an... odd choice.
Perhaps they became pirate lord via nepotism?
If you become a pirate overlord at 4th level, that doesn't mean there is automatically going to be a pirate a few levels higher waiting to try his luck.
Yeah, it kinda does. In the same way becoming the town's best gunfighter all but guarantees a challenger is going to show up in your average Western.
Characters don't walk around with their level over their heads.
This is a non sequitur.
Hell, his first mate could be higher level and stronger, but that overlord could have gained the respect of that first mate so that is why he is first mate and not captain.
This is also true.
But it does nothing to disprove my point. The most common and reasonable to represent a "pirate lord" is by using a higher-level character, not a lower-level one.
I'm sure I could whip up a low-level
10 year old pirate lord, some sort of watery Ender Wiggan and play them to my satisfaction. Still not the most sensible way to go about it...