"I'm from Gambia. There's no Kazakhstan Embassy in my country. So how can I get their Visa?"
The Kazakhstan Embassy in the UK is where you would apply for your visa ...
http://www.kazembassy.org.uk/en/pages/page/76
FAQ ...
"Q10. There is no Kazakhstan Embassy in my residence country, can I send my visa application to the Consular Section in London?
Yes, you can apply for a visa (by post or in person) at the Embassy of Kazakhstan in London, regardless your nationality or country of residence."
As Gambian passport holders are not one of the 47 countries eligible for simplified visa applications to Kazakhstan, you will need to find a company that will provide you with a "Letter of Invitation" (LOI). You will require one that will be at minimum a dual entry, 60 day visa (you must leave Kazakhstan within 30 days of entry before you are allowed to return, it will take longer than 30 days to conclude the business of getting married).
There is a company that I use for my LOIs, let me know if you want their contact details.
As for the question of your age, I would suggest that you ask your intended wife to go to the Office of Registration of Civil Events (ZAGS) in whatever city she is in and enquire there (they will give her a document with all requirements).
As for any other questions relating to marriage, there is a long thread on marriage to KZ citizens here ...
https://www.expat.com/forum/viewtopic.php?id=150350
