The mother is always the preferred primary informant for a birth registration, and there is no requirement for her (or any informant) to be of any particular age.
As far as paperwork - the registrar has to be satisfied that the birth took place in their district ( which is usually, but not always, confirmed by information from the hospital or midwife), and that the person attending is qualified to be an informant and able to give the necessary information.
The qualification for being the informant e.g. "mother", or "present at the birth" should also be shown on the certificate in col 7.