I think this would be helpful to you.

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Under normal circumstances, if your daughter is on dependant visa status and submits for renewal of dependant visa application prior to expiry of the existing dependant visa, she may be allowed to continue such dependant visa status (though after attaining 18 years of age). However, if she ceases to make such renewal application timely and apply for dependant visa only after expiry of the limit of stay, the application will be assessed according to then prevailing requirements, and hence there may be a chance that the dependant visa would not be approved for her due to having attained 18 years of age (which is outside the scope of eligibility for a dependant visa).

However, each case will be considered on its own merits. Without assessment on an application, I regret that we are unable to comment on any individual case or to make any conclusion at this general enquiry stage.
The entry arrangement for persons who wish to enter the Hong Kong Special Administrative Region (HKSAR) for residence as dependants is set out in the following website: 入 境 事 務 處
I hope you would find the information useful.