"Q. If at the time of my birth one of my parents was not a Kenya Citizen and I was born in Kenya, is it possible that I have dual citizenship? If the answer is yes up to which age?
A. Kenya Constitution Section 97 (1) (6) (a) states that:
Any person who, upon the attainment of the age of twenty-one years, is a citizen of Kenya and also a citizen of another country other than Kenya shall cease to be a citizen of Kenya upon the specified date (6)(a) that is: twenty-three (23) years."
And on the site that commenter linked to:
Q. What happens to a person who is under 23 and has dual nationality?
A. If such a person wishes to retain Kenya citizenship, he/she is advised to renounce other nationality before the age of 23 years. Failure to renounce results in automatic loss of Kenya citizenship after age 23.
I can't say for sure that that's right or that it settles the matter, but it certainly could be that this is a dead end.