Interesting thread. Reading through it resulted in me thinking of two completely different ways of dealing with NPC's of differing "experience".
Firstly a rules light method.
PC's probably won't be fighting NPC's (at least commoners) and if they do, won't usually be getting Xp for it (at least positive Xp anyway). For commoners, experts and any NPC's who have no real combat skill it's probably not necessary to go into the detail of assigning levels or determining BAB, Saves, HD and other combat stats. A simple summary of any attributes and skills of note and a description of the NPC should do. If the elder in village has lived a long, hard life and is an authentic wise man, does it matter if we assign a commoner level and fully flesh out all his stats, feats and skills. I would suggest that the following should suffice for any noncombatant.
Throbard Haltarsen
AC 8, HP 3, Int 14, Wis 17, Age 67, Knowledge (Farming) 15, Knowledge (Local History) 17, Sense Motive 9.
The eldest man in his small hamlet of Hulbeck, Throbard is much respected, both for his age and his acquired knowledge and wisdom. Throbard loves dispensing wisdom to those who ask, especially the young. Although his is apt to go off on disjointed stories from his youth, he has the uncanny knack to see to the heart of peoples troubles and give them a suitable answer, if they have the patience to listen through his stories.
Clunky Method
Funnily enough the process of thinking through a very simple method brought up a much more game mechanicky method for dealing with life experience.
Split "experience" into "combat experience" and "life experience". Experience in combat (or perhaps due to 'conflict' would be a better descriptor') contributes towards levels in the PC classes and also the more martial NPC classes such as Warrior and Adept. Many NPC's will never gain any 'conflict' Xp or at least not enough to ever gain levels in any 'martial' classes (I would suggest that with this method NPC's start at 0-level in regard to PC classes - needing perhaps 500 conflict Xp to reach 1st level).
Every character (both NPC and PC) also gain life experience in accordance with MavrickWeirdo's simple aging formula. This 'life' Xp would go towards levels in a generic 'NPC' class which only added to skills and had no effect on any combat stats. Therefore older NPC's (and PC's for that matter) would have a greater knowledge base than the young but not be any better at fighting or dodging fireballs (in fact they would likely be worse due to stat decreases with age). The skill pool that characters would be able to draw upon would be determined by the GM (or GM and player together in the case of PC's) and be flavoured to represent the characters life experience (or at least to represent their life experiences during the time period they gained the level). The number of skills per level would probably be 4, however restricting the skill choices to 'roleplaying' skills rather than ones which have a large effect on the game should balance this out.
For example a peasant who has reached the age of 43 is now a 5th level NPC. His skill pool will represent his farming and rural knowledge, perhaps some craft skills, some simple knowledges and perhaps a few social skills such as perform or diplomacy.
A veteran Mercenary who has just hit 43 will also have 5 levels of NPC. His skill pool will be more likey to draw on knowledge of politics and nobility, military tactics, gather information (come on we all know that Merc's gossip like old women), yet will probably not have access to any skills representing a rural lifestyle.
This method would mean that PC's wishing to start with 'veteran' characters would simply have to up their age. They would then start with more skills but also be closer to suffering age penalties (or perhaps even start with age penalties).
If any NPC is intended to have a small amount of martial skill, for example an aged peasant who served in the militia during his youth, simply add a level of Warrior. Age will take care of most of the benefits he gained from the experience - he will be weaker in combat than a 20 y.o. peasant but would maybe have the edge over another old man if an arguement came to blows.
In fact to give every NPC basic combat stats, their 0-level status in a PC class would give them the martial stats the Commoner NPC calss in the DMG.
Hope this makes sense.
Thoughts please.
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