The first recorded mention of the elder tree was all the way back in Old English during the eighth century CE, when it was spelled ellaen. Throughout the eons, it morphed in spelling, taking on forms like ellaern, ellarne, ellerne, and helren before adding a d in the fifteenth century with eldyr, eldern, and eventually elder, which became the standard by Shakespeare's time. Since the word has been muddling through the annals of history for so long, it's very hard to find its etymology (it has been reconstructed to Proto-Germanic elernaz, with the same meaning), but we do know that the d was probably added because people confused the tree with the alder, which is related to the birch and not a berry tree like the elder. The word elder meaning "older" is unrelated, but also comes from Old English, where it was spelled eldra, yldra, or ieldra. This would be from Proto-Germanic aldaz ("old"), which further traces to Proto-Indo-European hel, "to grow".
Hello! I'm Adam Aleksic. This year, I graduated from Harvard University with a degree in Government and Linguistics. There, I co-founded the Harvard Undergraduate Linguistics Society and wrote a thesis on Serbo-Croatian language policy, magna cum laude. In addition to etymology, I also really enjoy philosophy, trivia, vexillology, geography, board games, conlanging, art history, and law.