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They are not a child for this purpose. They are an adult and have to qualify in their own right for citizenship and meet all the requirements. If your partner is not the biological parent, I would suggest they complete separate forms.1. Can I add my 18yr old to my partner's application as a child?
LIUK and English B1 test is mandatory for ANYONE over the age of 18. So yes, your child needs both tests before they can apply for citizenship in their own right. Note there is no concept of 'dependent' for citizenship applications. Each application, regardless of age, must meet the requirements.2. If I cannot add my 18yr old to my partner's application, will He be required to take 'Life in UK test' to apply for naturalisation?
Yes. It is mandatory.3. Will He also require prove of English language to apply for naturalisation?