There is a bit of conflicting information about this herb. One version of the Lay of Leithian had Huan bring Athelas to Luthien in order to heal Beren. However, it's stated in LOTR that the Numenoreans brought it to Middle-earth. These accounts are not necessarily mutually exclusive, if the plant originally grew only in Beleriand.
One source says only the Rangers of the North remembered its true healing properties, while another states Elves also knew about them. Which one do you think is more accurate? Aragorn most likely learned herb-lore and healing in Rivendell, but he probably also learned many things while he was out in the wild. So either theory is possible.