For girls, this is simple enough - their first transformation comes with the full moon following their first period.
I'm not entirely sure what would be an equivalent marker for boys, though. Most of the pubertal changes I can think of for boys are pretty gradual. The best thing I can think of is "first ejaculation/wet dream" - would this make sense as happening around the same point as girls get periods?
(I realise I sound incredibly dumb for not knowing this, but...sex-segregated PHSE lessons, all I ever got taught in any great detail was FEMALE puberty. I have googled combinations of boys, male, puberty & adolescence, but I'm still not entirely clear as to what starts happening at puberty and what just starts to happen more - the sites are mostly geared to boys, who know what their bodies are doing pre-adolescence)