My understanding is that given the requirement under Regulation 11.16 of the Uniform Civil Procedure Rules 2005(NSW) for an Affidavit of Service to be completed by the person serving the legal documents, the person would need to be an adult (18 years or over).
In addition, further qualifications (e.g. Certificate III in Mercantile Agents) and experience are generally required if you are looking to become a licensed process server (and not just assisting a friend to serve their Statement of Claim or other legal document).