I don't think so there is anything wrong with that cause do you remember in Ancient times ,most of people married at around 14/15 but if they could hold the labour pain
what prevents that?, but of course both should be mature enough to do such a thing. but if they are already mature enough? then only traditional aspects interfere with that. but remember it's just man made bunch of rules.
don't measure other countries culture from our measurements.
If they need/could/ then go for that.!
consider the BIOLOGICAL aspect most of like those children got lower IQ score compare with others.