I am unclear if you are asking about the age of the proposed parent or the age of the proposed adopted person. There is no age limit on how old you need to be in order to be adopted and there is also such thing as an adult adoption. However, your question seems to ask about naturalization through adoption. I would strongly encourage you to consult an immigration attorney as what you are inquiring about may not be possible in accordance with the laws regarding immigration. Best of luck.
This answer is for general informational purposes only and should not be relied upon for your particular case nor is it intended as legal advice. I have not reviewed your case nor have I met with you and the answer to this question does not in any manner whatsoever establish an attorney/client relationship.