I too hate the 'wait till they're 23' mentality. I know no manager actually thinks like this but that's always how it seems to pan out. Thus, we always end up with patchy and inexperienced squads, and seems to celebrate 50 caps as if it is hugely impressive.
That said...
I really really want Hornby and Gilmour to make the grade for us, but they're not even on their clubs' benches for diddy cup ties or anything (bar a solitary run out for Hornby in the Europa League). At least Ampadu for Wales was getting a sniff of action at Chelsea.
If, say, Hornby got a few appearances for Everton under his belt in a short-ish space of time, I wouldn't be all for taking the gamble. As it stands it would seem like desperation to call him up now. We just don't know if he's an EPL-level striker or a Rotherham-level striker yet.