DX Wombat - even if littleflyer passed all except one exam on her 14th birthday and the final one when she's 15 years and 6 months old (i.e., passed them all in an 18 month period), they'd be valid for licence issue until she was 17 years and 6 months old.
But what she mustn't do is to pass them ALL before she's at least 15. Otherwise they would have lasped by the time she could apply for her PPL.
All exams must be passed within an 18 month window.
They are then valid for licence issue for 24 months from the date of passing the last exam.