Are they "level 50 only" content?
No, not at all.
The bosses are different levels, varying, depending on which planet they're on.
Like the world boss ''The First'' on Dromund Kaas, is made for a raid of people around level 12-15 or something like that.
The one we killed on Narr Shaddaa was level 31 and dropped loot around level 29-32.