Hard to say. Age isn't the primary factor with this position. A running back only has so many carries in him, be he 20 or 24 years old when the counter initiates. We don't know what that number is and it varies from player to player; however, if we are going by age, the decline typically begins at about 26 years. Elliott will be 21 when he sees his first NFL carry.
This isn't going to be a position by committee. If he's healthy to go every week (highly doubtful), I'm thinking we see around 300 total touches this year in addition to those plays he will be tasked with blocking or blitz pickup. Of course, I have no idea what the future brings, but if all goes well, 2016 should be Elliott's lightest workload heading into his mid-twenties. And if it isn't, then I am really wondering why the fourth overall pick was spent on him.
Come 2019, the best case would be Dallas and Elliott at odds over money, because a great runner will demand the the truck backing up for their second contract. What his physical state is by then is anyone's guess.
EDIT: To the original point, and I don't need feel the need to belabor this, but I am one who felt this was a misallocation of draft resources. Elliott is a fine runner, but the elite prospect many make him out to be. To me, Ramsey was the superior talent and a player who would provide immediate impact to an area in which this club is in dire need.
There were runners on the board in rounds two, three, and beyond who could unseat any running back on this roster. Is there a drop-off between Zeke and someone like Kenneth Dixon? Absolutely. But it's nothing like Ramsey and, say, Vonn Bell.