There is no age limit for applying for business registration as an individual business. Under the Business Registration Ordinance, it is the duty of a minor and his trustees (including guardians) to apply for business registration in respect of any minor carrying on a business (other than exempted business).
Therefore, even if an online business is under the age of 18, they are still required to apply for business registration in accordance with the law. In such cases, minors and their trustees (e.g. guardians) should comply with the relevant regulations and procedures in order to operate their business legally.
It is important to note that the specific laws and requirements may vary from region to region, and it is recommended that you check with your local business registration authority or professional for more precise and specific guidance.
https://www.ird.gov.hk/chi/faq/aibrq.htm