raids are level based.
this is shown on the screen where u select the boss to summon before u select it.
Tier I Apprentice (levels 1-20)
Tier II Adventurer (levels 21-50)
Tier III Master (levels 51+)
Tier IV Legend (coming soon)
(though i don’t know anyone of level 21 who is at adventurer yet or even anyone at 40 or below)
also, there are some raids that use stamina instead of energy. In those cases it becomes pvp or raids. personally i prefer these raids.