Welcome to the guide! If you're reading this, you're probably either a Warrior of Sunlight, seeking their own sun. Or maybe, you're the shadow of fear in the hearts of everyone coming to claim their humanity; The Darkwraith. Whatever you are this guide is here to help you come to terms the ranges of Cooperation and Invasions in areas.
Let's Get Started.

Unlike the original, Dark Souls Prepare to Die Edition had no matchmaking, but it had a set range of level that could summon people in. For the most part, the average player usually fell into a set number category based on the areas. The difference here now is that since weapon matchmaking is in place, there's just a bit more fluff to the equation. All in all, the Soul Level range is the most static change since Prepare to Die ever since Remaster hit.
There are supposedly three tiers of weapon MM. Your matchmaking is determined by your HIGHEST weapon upgrade. That means no storing the item in the box or dropping it or feeding it to frampt to drop your matchmaking. Once you get there, there's no turning back.
1-5 (Base Fire,Divine,Magic,Raw weapons fall here)
6-10 (Base Lightning, Chaos, Occult, Enchanted Fall here)
11-15 (+6 Fire,Divine,Magic fall here)
These are all REGULAR Upgrades.
For Clarification on things people may not know quite yet, Certain weapons that require Twinkling Titanite, Titanite Scale, and Dragon Scales are all considered before ANY upgrades to be a +5 regular weapon. All unique unupgradeable weapons are considered +5 (Dark Hand, Ivory Catalyst, etc.)
For a reference on Special Weapons; here's your Crunch.

• Base Special = +5 Regular
  
• +1 Special = +7 Regular
  
• +2 Special = +9 Regular
  
• +3 Special = +11 Regular
  
• +4 Special = +13 Regular
  
• +5 Special = +15 Regular

All in all, Fairly simple formula to follow for that.
Let's Get Started with the Earliest Locations.

• Undead Burg: 1-20
  
• Undead Parish: 10-30 +5 Weapon should be acquired by this point due to Andre being nearby.
  
• Lower Undead Burg: 10-30 +5 weapon since this is also one of the earlier branching paths.
  
• Darkroot Basin: 15-30 +5 Weapon. Butterflies, Hydras, and Stone Soldiers oh my.
  
• Catacombs: 20-40 +5 Although it's certainly had it's moments where early level people at level 10 will be here with a decent Strike weapon.
  
• The Depths: 15-30 +5 and into +10 territory if you've scrounged enough Large Titanite Shards. Access to Large ember changes this.
  
• Blighttown: 20-40 +5 and even +10 provided you've farmed hard enough for it.
  
• Sen's Fortress: 30-50 +10. Don't go into sen's without a +10 or close to it. You'll thank me later once you hit--
  
• Anor Londo. 30-50 +10 no exceptions. https://www.youtube.com/watch?v=-5yw0wHQx9A
• Painted World of Ariamis: 40+ +10 minimal. So you found this place huh? Welcome to hell; Toxin deathrattle hollows, Super Slap Hollows, and Birds with the thickest curves you've seen. This used to be an old Duel Arena for people due to it's beautiful enviroment; expect to get invaded by the time you're 60 or if you've been indicted; 40. Boss is optional.
  Lordvessel Achieved!
  At this point in the game the game splits pretty hard into 4 (or 3 if you went to New Londo Early) areas; I'm gonna do what I can to keep it simple by not dividing the sub areas too hard but i'll try to crunch it easily.
  
• Darkroot Forest: 40+ +10 minimal. Welcome to a covenant hotspot; I hope you're ready to fight other players if you're here. Dried Finger recommended if you're not so hot at PvP for another meatshield coop buddy.
  
• New Londo Ruins: Until you lower the water in New Londo; summoning is impossible. Post Water Lower is about 40-60+ depending on where you went. +10 weapon minimal and a +15 max since the Very Large Ember is here.
  
• Duke's Archives: 50-70 Crystal Hollows hit hard and have high defense against most everything (and the channelers don't help either). It sucks. +10 to +15.
  
• Tomb of Giants: 50-70 +10/+15 https://www.youtube.com/watch?v=1mrGdGMNsv0
  
• The Path To New Izalith (crunch): 60-80. Demon Firesage, Centipede Demon, DRAGON BUTTS, and the Bed of Chaos. Orange Glow everywhere. It's a good ole time.
  The End?
  
• Kiln of the First Flame: 60+ and no excuse for not having a +15 by this point. Prepare for NG+ on Gwyn's death.
  
• Oolacile: 60+ +15. Here we are. In the Past. Everything hurts,sucks, and will say you have a purdy mouth here. Prepare to Die.

I hope you all found the guide to be of some help to you. Most of the numbers are based on Prepare to Die back when I played and frequent Dark Souls chatrooms when people asked for help on the areas. It's very likely alot of the information is old but not entirely outdated. If anyone has any input or corrections they'd like to notify me of, please don't hesitate to tell me here. I'll keep an eye on it and update it to match the most accurate representation of how the current average is.
If you excuse me; I have some Jolly Cooperation of my own... To Ruin...