http://revach.net/article.php?id=743

Section: Halacha   Category: Tshuvos
Rebbi Akiva Eiger: Should You Say Birchas HaMazon Twice On Your Bar Mitzva Night?

If a Katan eats before nighttime of his Bar Mitzva night and says Birchas HaMazon, and only after nightfall the food is digested when he is already a Gadol, R. Akiva Eiger is Mesupak if he needs to Bentch a second time since the first bentching was only MiDiraban.  Now after nightfall he may be Mechuyav Min HaTorah because of "V'Savata"; and you will be satiated.

Rav Moshe Shternbuch (1:185) is bewildered by R. Akiva Eiger's notion that being satisfied alone could trigger a Chiyuv to bentch. He in turn is mesupak about a more common issue. Can you honor a bar mitvza boy who ate while he was a katan and became a gadol with leading the Mezuman since his eating took place while he was a katan?