>Does your damage output (at least as a melee class like
>warrior, assassin, etc.) increase at the 0 levels, meaning at
>20, then 30, then 40, then 50?

No, it doesn't. However:

1) You don't get full damage out of a weapon higher level than you are past some point; this is on something of a sliding scale. Thus, a level 22 character will probably get more damage out of a level 45 weapon than a level 18 character does, because he's 23 levels lower instead of 27.

(I don't know the numbers for this offhand. While playing mortals, I just try out each weapon to see which does more damage.)

2) The level 22 guy probably has higher skills in, say, sword and enhanced damage. He also might have more +dam from berserk, etc.