I've known other 17 year olds who were ready to start a family, and for whom waiting a year would mean bringing one less cherished person into the world. Which scenario is more common? Where should we draw the line? This is almost entirely subjective, and it's hard to fault a jurisdiction for inconsistency.
Let's err on the side that doesn't result in children being forced to marry pedophiles. Yes, the line can be subjective, but the vast majority of cases of child marriage are underage women being forced to marry men over the age of majority.
People not being able to marry at 17 doesn't stop them from waiting a year to be married. That isn't going to stop them from being patient, and won't stop them from having a child.
The existance of women who willingly marry early and have not been groomed since the age of twelve in no way negates the existance of women who have been groomed from a young age into marriages they don't want and rush to leave.