You can play a game with friends from the earliest levels, it's just not necessarily a difficult early game because not everyone is looking for the challenge you are. You can do Palace of the Dead from level 17. It's up to 4-person content that goes up to floor 200 and floor 101-200 is the challenging version, which even full parties sometimes cannot do.

There is actually end-game content at quest intervals of level 50, 60, 70 and 80 which represent the maximum levels of each expansion. For the older content to be challenge you need to open Duty Finder and select "Minimum IL" and "Silence Echo". The challenging versions are known as Extreme, Minstrel's Ballad, Savage or the coil raids.

You can boost to the level 50 quests a lot cheaper for just $8 with Tales of Adventure: A Realm Reborn.