Can a child (preteen) heart be donated to an adult?

Rarely: I am not a transplant expert, but from what i know it depends on the relative size of the donor and the recipient. If the donor is larger size person, and the adult smaller in size, it is possible. But i would think that many times if the tissue match is ok, the would likely keep a pre-teen heart to match with a like age recipient.

Yes: Hearts are matched by blood type and body size. A preteen or teenager's heart can be matched to a small adult - say someone that's 4-5 feet tall and weighs 80-100 lbs.

Possible, unlikely: The compatibility of a recipient and donor depends on many factors. Size of the heart is one. In general, a pre-teen's heart is unlikely to be of adequate size to be transplanted into an adult. But the specific match depends on each individual donor and recipient.