I don't consider this scenario canon, in fact it explicitly ISN'T, but... a Meng Yao who is actually a year OLDER than Zixuan but realises after being thrown down the stairs that being a threat to his half-brother's inheritance would not only dash all his chances of acceptance but probably get him killed if the wrong people in lanling found out. So he uses his baby face to pretend he's 2 years younger than he actually is, a year younger than Zixuan instead of older. (Jin Guangshan genuinely forgot when he slept with Meng Shi so he buys the lie)

I mean just... secret true heir of lanling Meng Yao and no one except he knows it. What a concept.