Maybe Elder Trees suffer from a magically induced aging disease?
There are disorders in humans that make people age far more rapidly than they should. Mentally you're a teenager, but your skin or hair act like you're in your 60s or 70s.
The peculiarity of this species of tree is the same thing.
Even though the tree is technically young, it manage to age itself rapidly to grow (keep in mind that trees grow from sapling to full size in a matter of hours, where in real life, that process takes years).
Or perhaps it takes a page from the Elves.
Time to it is stretched out in a way that it acts like centuries pass for it in a matter of days.
Once it is fully grown, to us, it looks old and aged.
09-Nov-2016 14:55:27