MrsArcola Cxx

Hi hope you can help. My child has a CTF maturing this year (18th birthday) and they plan to start a SIPP at 18 with some of the money . In education, works PT (does not pay tax). I have read that previous years allowances as a non tax payer can be used? Can this apply for under 18 years please (child)? and how many years could be used? I understand £2880 tax yr 24/25 can be invested in SIPP. ie Can a further £2880 be invested for yr 23/24 in retrospect? Any assistance would be greatly appreciated.