The affix ini (in) comes in the sense of matup after an ordinal numeral when age is denoted.,
Thus पञ्चमोऽस्यास्ति मासः संवत्सरो वा = पंचमिन् 'five months or years old', as पंचमी उष्ट्रः ॥ Similarly नवमी, दशमी &c. By 5.2.115 the affix इनि would have come after these words; the present sutra is a restrictive rule teaching that ठन् does not come in this sense.
Why do we say वयसि 'when age is denoted'? Observe, पंचमवान् ग्रामरागः ॥,