I don't think anyone is expecting him to play Super Rugby as a 17 year old. He isn't even joining the Reds until next year's offseason. As said above, its just patronizing to think he was only signed because of his highlights reel and his surname (Alex Horan isn't capped for QLD by the way). As has been documented on this thread numerous times, QLD Rugby (for numerous reasons) have lost their next generation of flyhalf options that have been coming through the system for a few years - Lucas, Lolesio, Mason, Pasitoa, the Gordons, etc. The cupboard is pretty damn bare from this year's GPS comp.
To have someone, who clearly has some ability of the video is any indication, but with the far superior England academy structure behind him, is a bloody damn site more positive a signing than hoping one of the scholarship holding contract chasers come 'home'.
As for your arbitrary 'perfect age to debut' I'd have expected Carter Gordon to make his Super Rugby debut for us next year - at 20. After we managed him pretty damn perfectly since he left school (and even while at school). We invested in him, and now Melbourne get's the rewards. What makes you think this isn't another investment?