In modern Kenya, policies assure that proof of age is much more accurate than was the case before 1990. I'll bet that Isaiah Koech has as accurate a proof of birth document as the average U.S. citizen. Here are some concrete facts to consider:
Republic of Kenya National Registration Bureau:
http://www.identity.go.ke/index.php?option=com_content&view=article&id=88&Itemid=114
FAQ
Q13. Why should I not be allowed to assess the age of my child?
Rule 4(2) (Rule 4(1) & (2), Principal Rules of the Registration of Persons Amendment Rules 1989) forbids that and prescribes that where a person attends before a registration officer for the purpose of having himself registered under Section 6(2) of the Act, the registration officer shall demand proof of date of birth or apparent age by production of the birth certificate or age assessment certificate issued by a government medical officer of health or a baptismal certificate issued by a minister of a recognized religious organization immediately following his birth or some other evidence acceptable to the Principal Registrar.
Refugees International:
http://www.refugeesinternational.org/policy/field-report/kenya-national-registration-processes-leave-minorities-edge-statelessness
In Kenya, a national ID is the hallmark of citizenship
Under Kenya’s Registration of Persons Act, citizens 18 or over must register with the National Registration Bureau and obtain a national ID. Failure to do so is a crime.
ID cards are required to register to vote, obtain a passport, purchase property, open a bank account, conduct business, seek employment, access higher education, enter government buildings, and get married.
The registration process relies on the Constitution of Kenya and the Kenya Citizenship Act, which provide that citizenship can be obtained through birth, descent, registration, or naturalization.
The registration process requires proof of age, usually shown by a birth certificate, and proof of citizenship.
Proof of age (birth certificate, baptismal card, school leaving certificate)
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Kenya isn't nearly as primitive a nation as some here would have us believe.