Another way to do it is to make a level difference cap.
Level 40 vs level 45 okay you can win with skills. If you have a bigger difference, you can't attack.
That will prevent the high-level players to go to the starting zones and mass murder everyone they see, or gank you later on.
Any level cap on PvP handicaps many of the social dynamics that make an MMO an MMO. Scars of Honor claims to be an "Old-School" MMO. Level capping or Toggling will kill that claim. If you want to protect low level players do it through GAME-PLAY and not arbitrary systems that prevent players from playing. Place guards in lowbie areas, have an alert system that notifies when enemies may be in the area, make it easy for lowbies to ask for help from fellow faction members, use terrain and world design to make it difficult for factions to invade each others cities or corpse camp, etc...
If you take away the danger, player friction and fear of loss you haven't created an MMO...you've created a boring "Safe-Space" that people will soon leave.