Interestingly neither LASORS, SRG_FCL_19_A, nor the ANO actually answer the question as asked. (unless I missed it in the small print somewhere)
All three documents agree that you have to be 17 in order to apply for the licence to be issued. LASORS goes further and states that
An applicant for a JAR-FCL PPL(A) shall be at least 17
years of age but some of the required qualifications for
the grant of the licence may be gained earlier.
Applicants should ensure that any qualifications gained
earlier will still be valid at the time they plan to apply for
the grant of the licence.
(emphasis mine)
but helpfully doesn't then state which qualifications may be granted earlier.
My reading of them is that you can do everything up to the application prior to your 17th birthday, then send off the application on the day itself, but its probably worth checking to see if your CFI or examiner can give a definitive answer to the question. Alternatively give the SRG unit a call (and let us know what they said!)