Boosting will no longer net the 3x xp in the near future (per Blue's post). I would still suggest not linking the 70 so you can run your alts thro instances to complete quests/get loot but if you dont care about that then feel free to link any accounts.

In my experience (don't know about anyone else), regardless of the levels of my alts (currently 43 44 44 45) all 4 of them get the 3x xp when killing mobs/completing quests that are not grey to them.

Also, regardless of wether the linkee has the same quest or not I still get 3x xp for turning it in aslong as the linked partner is present.