The First Age began with the rising of the moon. This was after the Noldor left Valinor in exile, after Mortgoth destroyed the trees, and well after the Elves awoke in Cuivienen. The Awakening of the Elves was after the era of the Two Trees has started, but before the First Age began.

Worth noting the Belinda reveals was taken on May 24th, not April 12th, the date of transmission, to sync the viewer's and the Doctor's timeline. The penultimate episode, Wish World, airs May 24th and I surmise The Doctor and Bel return to Earth in that episode, thus finally synching our timelines.