In regard to my posting above, I received on my email a request for
sources. I don't have any sources off hand, but around 20 years ago I had a
student who was adopted and becoming Bar mitzvah and at that stage, he
would most likely have declined; I was told in discussions with other
members of the staff that were more personally involved that there was a
Psak (Halachic ruling) that he can wait until 16. Being that this happened
so long ago, I don't remember any more who gave this Psak. My main point is
that when one is in a situation such as this, one needs to be in
consultation with proper Halachic authorities.
Eliezer Wenger