How do I alphabetize the photos in People?

By default they are sorted by oldest first. The order can only be changed manually by dragging the various square face items into the order you'd like.

However, in doing so be very careful to not drag one face onto another and let go. That will merge the two groups of faces into one - the one that is on the receiving end of the drag and drop. If that happens immediately try using the ⌘+Z keystroke to undo the merge.