वृद्धस्य च

Operational rule vidhi

ज्य is also substituted for वृद्ध when the affixes इष्ठन् and इयसुन् follow.

Word by word (padaccheda)

Target ṣaṣṭhī · ekavacanaIndeclinable च avyaya

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

Dotted words are technical terms, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

च

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

ज्य is also substituted for वृद्ध when the affixes इष्ठन् and इयसुन् follow.
Thus ज्येष्ठः 'the oldest', ज्यायान् 'the older'. By 6.4.157, वर्षि is also substituted for वृद्ध; and we have वर्षीष्ठः and वर्षीयान् also. The word वृद्ध though not a gunavachana, takes these affixes by the implication of this sutra.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.