There are a number of ways to become eligible for UK citizenship. It can be either by:
The applicant can apply for their first British passport at any time – the applicant does not need to do it before they reach a certain age.
British citizenship can only be inherited through a parent by the first generation born outside the UK in almost all circumstances. Different rules apply to children born in British colonies and territories.
The UK recognizes dual citizenship. This means that any other citizenship you have will not conflict with your claim to British citizenship.
However, if the applicant claims British citizenship, he/she needs to make sure they do not conflict with their original citizenship as nationals from certain countries, such as Germany, automatically lose their original citizenship if they naturalize or claim another citizenship (under most circumstances).