You might be able to plunder some ideas from the "Lost World" campaign created in DRAGON Magazine's Dungeoncraft between Issues 280 and 290 (
http://www.wizards.com/default.asp?x=dragon/dungeoncraft).
If you have or can access the 2E book Player's Option: Combat & Tactics, look for the Stone Age Weapons list there. If you don't and can't, try this to start, pruning and grafting to taste:
Shortspear
Club (Cudgel, Knobkerrie)
Bolas
Net
Sling
Boomerang/Discus
Javelin (with Atlatl; also Harpoon)
Blowgun
Hand Axe (Tomahawk, Adze, Hatchet)
Quarterstaff/Staff Sling
Dagger
Shortbow
A fantasy Stone Age might also include stone/bone/wood/sinew equivalents of the following (some of these items may have existed in Earth's Stone Age):
Warhammer
Maul
Greataxe
Shuriken
Darts
Shortsword (Macuahuitl
)
Composite Shortbow
Longbow
Composite Longbow
Armor in the Stone Age is probably most logically limited to studded leather or lighter (perhaps some primitive genius sews flat river rocks into his leather coat). A bone shirt would probably provide equivalent protection.
Most if not all of character classes ought to have Wilderness Lore as a class skill. In a typical Stone Age, Wilderness Lore includes the skill to salvage all the usable bits from animals (bones, skins, sinews, fatty tissues, meat, entrails, etc.) and the skill to harvest edible bits from naturally occurring plants. Musical instruments have limited variety, so Perform choices will be limited. You will need a new list of Craft and Knowledge skills as well, and you might consider making Craft a class skill for everyone as well. Look over the other skills and omit the ones that depend on unavailable societal norms and/or technological progress.
Decide how sophisticated you want religion to be. Many of Earth's early religions were very sophisticated indeed, though despite this we modern folk tend to think of Stone Age people as fearfully worshipping natural forces and observable phenomena as divine manifestations.
What magic there is in the world should be very subtle. Consider making the "special effects" of any "flashy" spell more low-key: instead of a column of flame,
flame strike simply creates an invisible area of intense heat. Consider "throwing the bones," or something similar, as an added Divine Focus for any Divination spell (increases 1 action casting times to 1 round). Do people think magic is a good thing or a bad thing? Do people trust spellcasters?
Strictly enforce the rules for environmental exposure, starvation, and thirst. Are resources plentiful or scarce? Do people fight over food or do they trade delicacies? What are villages like? Will you do without inns, taverns, and shops? (You might consider it, unless there is an uncharacteristically large city in the world.) How do the PCs acquire things they can't make themselves?
Note that the most playable period of the Stone Age may very well be the Neolithic or New Stone Age (c. 8000 to c. 3000 BC). The wheel has been invented, farming and pottery are established, a number of animals are domesticated, and some tools and weapons are polished and ground into shape instead of being chipped. People live in fairly sturdy buildings, having abandoned caves millenia ago. Neanderthal Man is long gone. Writing may have been invented and the first metal tools may be available.
Why do the PCs adventure? What's their shared background? Are they a group of tribal hunters? Are they all that's left of a once-thriving tribe, seeking revenge on whomever or whatever wiped out their families? How does society view what the PCs spend their time doing?
The more I think about it, a Neolithic Age where dinosaurs still roam (perhaps replacing herd mammals as a food source?) would be a cool place to adventure. Most of the things that D&D players take for granted will be unknown and absent!
Finally, a quick list of URLs:
http://ragz-international.com/stone_age.htm (general historical progression)
http://www.primitiveways.com/ (Stone Age crafts)
http://www.delanet.com/~ftise/stoneage/stoneagelinks.html (Paleolithic or Old Stone Age)
http://www.nba.fi/NATMUS/Julkais/Prehist/text.htm (historical periods in Finland)
http://www.visitdunkeld.com/stone-age.htm (Neolithic Scotland)
http://museums.ncl.ac.uk/flint/menu.html (hunter/gatherers)
http://www.bulgaria.com/photos/web/culture.pdf (a Neolithic excavation)
HTH. HAND.
