The Manifestation Age is the general age range of natural Elvin manifestation. Mr. Forkle mentioned this age range when telling Sophie that she is almost past the manifestation age and will not be able to manifest with any more abilities soon. Based on how early most of the Elves manifest and how late, the natural manifestation age is presumably from 12-16. It can happen later or earlier than this range, but it is very rare and unlikely.
However, in Sophie's case, her Telepathy manifested when she was five, which is younger than any Elf, but that is because she hit her head and Mr. Forkle triggered her Telepathy and planted the necessary information in her head. Keefe also manifested an ability in Legacy using stellarlune, which implies that stellarlune may be a way of manifesting an ability.