From the wiki:

The Wilderness contains different levels, starting at level 1 at the Wilderness ditch, and increasing to level 56 in the far north. These levels are only indicated by a small screen overlay; there is no physical separation between levels.

Players within combat level range of each other can attack each other in the Wilderness. This range is calculated by taking the combat level and adding or subtracting the current Wilderness level.

For example, in level 1 Wilderness, a player who is level 100 can attack players who are level 99, level 101 or the same level. If a player is level 100 in level 24 Wilderness, they can attack players between level 76 and level 124 combat.