If you are looking to train an 18 year old as a long term high level fighter you want to avoid physicals until you are readying them for fighting. Physicals train at the same speed all career long, but the other skills train fastest when the fighter is young. Using up the fastest possible training for physicals or time lost from fight recovery can slow a fighters progress and in some cases prevent them from reaching their highest levels. Mind you this does depend on how often you do these things, as a single fight won't cripple a fighter and you can gain some valuable info to help you determine if this is a project worth putting time and money into.
Don't spar early as sparring gives some skill gain to associated secondaries, so if you spar MT for example you would have skill points bleeding into kicks, elbows and knees at least. This again could have you reaching the skill can before you max every skill you had planned on (which usually is 7 or 8). Boxing is about the only one you can get away with early, as the secondaries are generally critical for any MMA fighter.
Sparring one skill all the way and switching, or spreading into the varying skills you want would depend on your plans for the fighter I'd say. Both are workable for a long term project, though if you wanted to take the odd fight along the way shoring up any glaring weakness might be something to consider.