I've never heard of the 18+ rule, but I would imagine it would depend on the individual rules of whichever keuring/show/property/etc you're at.
I've heard only praises to high heaven about how gentle Friesian stallions are. I remember one story where an owner's daughter got out of the house and ended up in the stallion's pasture... When the mom/owner found her, she was sitting on the ground playing with the stallion's feet, the horse being as gentle as could be.
But of course, there will be exceptions to every rule, whether the exception is a gentle stallion or an aggressive stallion.
I know approved Friesian stallions are also judged on their stable manners, along with other criteria, so that would promote the breeding of gentle traits.