Poems by Heart is one of my favorite apps and Im so glad they finally added Yeats. In response to YeatsianMe, it looks like they used an earlier version of "When You Are Old" in which Yeats did write the line with the word "will" included (as seen in the Project Gutenberg collection of Yeats poems). So this version is also correct, just an earlier edition published before Yeats revised his work.
BookDiva24 about Poems By Heart from Penguin Classics