Even though, in some extremely rare cases, a five year old girl may be physically mature enough to become pregnant, that doesn't mean she's emotionally mature enough to make a decision on her own. So yes, an adult should make the decision for her.
Okay, so just to get this on the record: MarkD is OK with forcing a 5-year-old girl to give birth.
Not emotionally mature enough to make the decision, but emotionally mature enough to give birth.
I misunderstood this trollish question. Backing out of this ridiculous discussion.