As a plant grows to an elder you can harvest it several times by walking up to it and clicking 'X'. Once you have harvested an elder it is gone forever. You can not revive it. There is a slim chance of getting a seed that was the same kind of the plant, but a greater chance in getting a new seed. I know this due to my BoomShroom. I had a BoomShroom named Boomy and it get into a elder. Sure enough I had harvested it many times before and got some seeds and Gardening XP. It was a sad sight to see the guy go but now he rests in peace. A plant will not grow back. You can get Reagents from harvesting and treasure cards too. I got a few of these things from Boomy.I know it is silly to name a plant (especialy in a game ) LOL. :-) Well I hope I helped. Have fun gardening!!!
~Elizabeth Heaven, Death school, level 26
P.S. Oh sorry I didn't answer your question. If you never harvest an elder but take care of it the elder will stay as an elder until death or harvest. It is best to just harvest it though. ;)