Well,According to Jagex COO, Phil Mansell, this announcement isn't coming out of the blue - in recent player surveys, 90% of active players and 64% of former players made it clear that they were interested in playing a mobile version.
As far as release dates goes, Old School RuneScape will be getting a mobile version sometime this winter, with the regular game's mobile version slated to arrive soon after, in early 2018.