What levels does Tripwire suggest for each difficulty?

Per the guide, Hard is level 6 to 12, Suicidal is level 13 to 25, and Hell on Earth is level 25.

Why does a low-level player hurt the team on Hard?

Perk levels give quantifiable bonuses, and ZEDs gain health and more dangerous behavior at higher difficulties. A character below the server's level contributes less and makes the game harder for others.

What if I am a skilled player but low level?

The author argues most people under level 10 are not that skilled and should get familiar with the game first; he asks you to consider others before choosing a server.

Running a Server

KF2: Under Level 10? Do Not Play on Hard

This short opinion guide argues that choosing the right server difficulty is the first sign of a good Killing Floor 2 player. Its title says it all: if you are under level 10, do not play on Hard.

The argument

Do you think you are a good player? The author means two things: good skills and good community play. Playing a low-level character on a Hard or harder server affects your performance, however skilled you are. The advantages of perk level are quantifiable on the perks screen, and ZEDs have more HP and behave more dangerously at higher difficulties. A character lower than the server's difficulty calls for cannot contribute as much and makes the game harder for everyone else. In a game where contribution decides whether the team lives, that is critical: if you are too low a level for the server, you kill your team.

Ask yourself why you are there. Are you on Hard because Normal is no longer a challenge and your skills are ready? Because every Normal server is full or laggy? Or to bleed XP from more experienced players? Think about how your choice affects others' enjoyment.

Author's comments

The author had over 600 hours in each of Killing Floor and Killing Floor 2 and noticed more experienced players and level 25 characters as the game was rebalanced and refined, with fewer inexperienced players around. Killing Floor's mayhem has subtlety: knowledge of maps, ZED behavior, perks, loadouts, team tactics and coordination matter, but the hard numbers of perk level still cannot be ignored. Even if you are skilled, most people under level 10 are not, and they need encouragement to learn the game before progressing to Hard. If you are an inexperienced character, stick to your skill level.

Official guidelines from Tripwire quoted in the guide:

Difficulty	Level
Hard	6 to 12
Suicidal	13 to 25
Hell on Earth	25

See the related new player survival guide for another level-to-difficulty rule of thumb.

FAQ

What levels does Tripwire suggest for each difficulty?: Per the guide, Hard is level 6 to 12, Suicidal is level 13 to 25, and Hell on Earth is level 25.
Why does a low-level player hurt the team on Hard?: Perk levels give quantifiable bonuses, and ZEDs gain health and more dangerous behavior at higher difficulties. A character below the server's level contributes less and makes the game harder for others.
What if I am a skilled player but low level?: The author argues most people under level 10 are not that skilled and should get familiar with the game first; he asks you to consider others before choosing a server.