才4 leans on a number and grades it as too small (才十一歲 = only eleven). 才5 leans on the verb and presses a stance against a contrary view (才不敢呢 = no way he'd dare). One judges an amount; the other digs in against what was assumed.
才1 and 才2 mark when the action arrives — held back, later than expected, only after a condition. 才5 marks no time at all: it presses the speaker's verdict against an assumption. Success: 才5 reads as a firm rebuttal. Failure: it reads as 'not until / only then' — wrong point.
他不敢穿。(flat statement) → 他才不敢穿呢!(emphatic: no way he would, pushing back on the assumption)
他才十八歲不敢穿呢。(才 stranded on a number, wrong sense) → 他才不敢穿呢!(才 leans on the verb to carry the stance)
他才不敢穿了。 → 他才不敢穿呢!(close with 呢, not 了 — 才5 seals a stance, it does not report a change)
English carries this with stress and words like 'no way' or 'as if'; learners leave the Chinese flat or reach for 不 alone. 才 plus a clause-final 呢 is what loads the rebuttal, with 才 fixed before the verb.