Jamieson, Fausset and Brown on Genesis 21:8

8. the child grew, and was weaned--children are suckled longer in the East than in the Occident--boys usually for two or three years.

Abraham made a great feast, &c.--In Eastern countries this is always a season of domestic festivity, and the newly weaned child is formally brought, in presence of the assembled relatives and friends, to partake of some simple viands. Isaac, attired in the symbolic robe, the badge of birthright, was then admitted heir of the tribe [Rosenmuller].