I suspect that the young age in the casting call for Aragorn might be connected to possible future film projects, specifically one or more movies exploring his previous journeys and errantries (Third Age 2957 to 2980 and perhaps beyond in the continuity of Tolkien's legendarium).
Sure, that's one of basically two options for the second film which seem the likeliest. But I just wonder if they will also have the "Mature" Aragorn (as opposed to "Old Elessar") of Mortensen's for scenes set at the time of Fellowship of the Ring.