The first season of Legends of Tomorrow gave us an older Oliver Queen in one of the series’ best episodes, so an older Barry isn’t outside the realm of possibility.
It’s worth noting that we still have absolutely no idea how Barry Allen factors into the narrative of Legends of Tomorrow’s next season.
Given The Flash’s ability to travel through time due to his use of The Speed Force, the Barry that sends the message could be an older Barry from 2056.
His voice does sound deeper and more weathered, but that could easily be explained away if needed.