I vaguely recall they changed the professional terms rule to be age 18 for anyone of non-EU status across Europe, though I may be wrong. I do recall the issues with work permits in Britain in those days and possibly still to this day although more relaxed now. Kirovski did remain at the club for four years in the academy and reserves without getting a work permit. I think the rule was changed to allow for NT 'B', U23, U21, etc. caps to count as it was difficult to expect a teenager to gain 75% full NT caps in a two-year window. There was another kid that joined Barcelona a few years back, De La Fuente.
From what I recall from 20+ years ago, some countries permitted pro contracts before age 18, like Germany and Netherlands and I think Turkey (as Gus Kartes signed pro terms at age 15 in 1996). But I recall others like France, Italy, and Spain mandated age 18/19 and that is one reason the club were able to lure Pique and Rossi away, amongst others - could give them pro terms on EU status before age 18.