The affix atṛn comes after the verb jṛ 'to grow old' with the sense of past time.,
As जृ + अतृन् = जर् + अव् 7.3.84 = जरत्, 1st. sing. जरन्, 1st. dl. जरन्तौ, 1st. pl. जरन्तः 'grown old' (VII 1. 70).
By the rule of वासरूप 3.1.94, the Nishtha affixes also come after this verb. As जीर्णः and जीर्णवान् । जृ + क्त = जिर् + त 7.1.100 and 1.1.51 = जिर् + न 8.2.42 = जीर्ण 8.2.77 and (VIII. 4. I) 'grown old'.,