We (Kennoway Star Hearts U20's) were informed that because the players in question are signed with the 1st team then it is ok. We have 15 signed players who are aged 16 or 17 and we tried to get around the restrictions of not training or playing with the rule of them being Under 18. However, because we are affiliated with the Lowland league at U20's then we are not allowed, but our players can play with the 1st team due to them being the parent club and that they are signed.
Hope this answers the question