The law cares only about legal age, so birth certificates and the like. That's how legal lolis can exist in our world - they simply grew slowly or just didn't grow very large. So their bodies might seem/look like they are young, but their legal age is, well, of legal age.
So in this case, if she says she's 29 and there's no documentation to deny that, then it's fine. Unless it becomes a legal case, in which case the lack of documentation would probably mean a doctors examination to approximate an age through X-rays of some bone growth or something, but such methods are fairly imprecise so while they probably wouldn't agree she's 29 they probably wouldn't be able to say for certain that she's illegaly young to marry.