Hi Faizan,
First, the study materials state that the minimum age is 18 so this is the correct answer and what you should learn for the test. Basically, if in doubt learn the study materials as written, even if you know they are wrong!
There are quite a few other places where the study materials are now wrong and this is because they were last updated in 2007. Picking one at random for instance - the Northern Ireland Assembly was actually reinstated in 2007 but the study materials say that it is currently suspended. Other examples are that the minimum wages have changed several times since, as have TV license fees and the cap on students' tuition fees.
The UKBA have advised that there shouldn't be any questions on these topics that are out of date. That said I would very much recommend you learn the study materials, even the bits that are out of date.
Hope this helps
George
