http://revach.net/article.php?id=1532

Section: Halacha   Category: Tshuvos
Chelkas Yaakov: Children Born in the Eighth Month Are Ketanim Until 20 Years Old?
In a novel tshuvah the Chelkas Yaakov (YD 179) suggests that a child born in the eighth month may not become Bar/Bas Mitzva until the age of 20.  The gemara in Yevamos says that Rebbi Avahu said that children born in the eighth month we don't do anything with them until age 20.  According to Rashi, Rebbi Avahu is suggesting that they are a Neifel.  Therefore since we paskens like Rebbi Shimon ben Gamliel that after 30 days a child is not a Neifel we don't hold like Rebbi Avahu.  However most Rishonim hold that Rebbi Avahu is talking about the age when the child becomes a Gadol and is no longer a Katan.  In that case we pasken like Rebbi Avahu and an eighth month baby is a Katan until the age of 20.

Nevertheless says the Chelkas Yaakov this is only if we know for certain that the child was born in the eighth month.  However despite the doctor's opinion and even with x-rays and sonograms there is still a possibility that the baby was actually born in the ninth month.  He concludes that since Rebbi Avahu's halacha only applies to a child that is 100% guaranteed born in the eighth month therefore we can consider an eighth month baby a Gadol even at the age of 13.

Disclaimer: We try to convey the Tshuva to the best of our ability. We admit that our understanding may not be accurate. Please also understand that this Tshuva may not be the final word on this topic. One should consult a Rav before drawing any conclusions.