A lot depends on the needs of the team. If you’re looking at it from purely development perspective, rather than say an injury/form crisis, then the balance of the team is important.
If the team is struggling to create chances, chucking a young lad up front does him no favours. However if the chances are being crested, I think it’s good to give a young striker the chance (especially if the senior forwards aren’t taking them)
Central midfield is probably the toughest area to break through physically, young players need to be strong enough to cope with the midfield battle that most Premiership games have.
Ultimately it comes down to attitude and being physically ready, more than ability. Jason Cummings was full of confidence and swagger, Ryan Porteous was strong and up for the challenge. Fraser Murray probably has more ability than both but hasn’t broken though (yet), that’s telling.