Blizzard has again updated the minimum level requirements for Timewalking events in the first Patch 10.1.7 Release Candidate build.
Last week, the minimum level requirement to participate in Timewalking was changed to Level 45. This week, we have even more changes, as spotted by MrGM.
Burning Crusade and Wrath of the Lich King Timewalking require Level 30.
Cataclysm and MoP Timewalking require Level 35.
Warlords of Draenor Timewalking requires Level 40.
Legion Timewalking requires Level 45.