Many MMOS have a either 1. A hybrid system where new players start at level 1 with no gear and veteran players start at their previous level. The game would detect their level and only allow them to play at the appropriate level until they've completed the mission, thus unlocking difficulty tiers. 2. A booster for new players to grant them min level requirement and one set of gear to play the content at veteran levels. TTK offered this option via Spark of Light which boosted players to level 25 and fully leveled one subclass.