An adopted (non-Jewish) child must be converted by a bet-din. The child
undergoes immersion in a mikveh (a father in a bathing suit can go in with
the child and if the child is an infant the father only releases the child
for an instant while under the water.) The child must later affirm that
they want to be Jewish when the reach the age of obligation for mitzvot.
This is usually accomplished through the decision to have a bar or bat
mitzva. Therefore, a bar/bat mitzva has a different overtone for an adopted
child. At the age of 12 (for a girl) or 13 ( for a boy), they get to choose
if they want to be Jewish whereas a child born of a Jewish mother is Jewish
no matter what they choose. This was no small issue for my two oldest
children who thought long and hard before deciding to have their bar and
bat mitzvas.
Froma Fallik