Not sure what the cut off is. I’m pretty sure it’s 49 for there to be an empowered version.
Is it like 74 or so for a mythic? or do even lvl 75 items have mythic versions. because that would be super 
According to the item database, it looks like 82 is the earliest for Mythical items.
Items in the 50-75 range can all have a mythical version. Lots of epics don’t however, beware (and tell devs you want them).
Cool good to know 