Your instincts are fine on this. In general if players are to make it, they play by the time they are 18/19. However one key point in Broadheads favour is that he has looked very comfortable so far in pre-season and is yet to be bombed out.
You can end up doing a lot of analysis on young players and who might make it, but in the end it comes down to whether the first team manager fancies them or not. I know it sounds obvious but Silva sees something in him and he has done well so far in pre-season.
He is a long way off being our most decorated young player to come through. Substantially behind the two you mentioned above and even Kieran Dowell. It's also worth noting Dowell looked similar good in pre-season matches and around this time of pre-season would be bombed out. That's the challenge for Broadhead. Stay in the team for as long as you can, even if it's just pre-season games. The more you stay in, the more chance you have to impress the manager.
As a player I have been impressed. He seems to have quick feet and a good brain in the final third. Monaco are no mugs and he looked ok there. Still a long way to go mind.