I'd probably go with amount of control one person can have over another. If both people are materially and independently secure (housing, food etc), and neither's position in life is contingent on the other's approval (e.g. teacher/student, boss/worker etc), then its probably fine. Or at least, more likely to be fine than a hard and fast age rule. There's a lot of psychological fuckery that can still go on, but how would one assess that from the outside?