I am a 18 years old Hong Kong citizen, can I apply for BNO

Probably not for BN(O) status/passport, unless you already have BN(O) status. Registration for BN(O) was time-limited before the 1997 handover and cannot be newly applied for now .

• If you are 18 now, you were likely born after 1997, so you normally cannot newly register as a BN(O) citizen .
• You can only apply for a BN(O) passport if you already hold BN(O) status .
• However, you may be able to apply for the UK BN(O) visa if you are the child of a BN(O) parent, are aged 18 or older, and were born on or after 1 July 1979 .
• BN(O) visa eligibility is different from “applying for BN(O) status”; the visa route can include eligible family members of BN(O) status holders .

So: you likely cannot apply to become BN(O), but you may be able to apply for the BN(O) visa if one of your parents is a BN(O) holder.