From: Rabbi Akiva Eiger
To: Unknown recipient
When an expert scribe brought his son to assist in writing STaM in Friedland, Rabbi Akiva Eiger immediately protested, demonstrating from the Magen Avraham that since a minor is disqualified from writing STaM by Torah law, Rava's presumption of maturity at age thirteen is insufficient and actual physical examination for signs of maturity is required. The teshuvah then expands into a wide-ranging analysis of related questions including the number of witnesses needed for the examination, retroactive validation of STaM written before examination, and the scope of the concern that signs of maturity may have fallen out.
Source: Shu"t Rabbi Akiva Eiger §7