अत ऊर्ध्वं चतुर्वर्षः पञ्चवर्षो वा कर्मण्यः पूर्णप्रमाणः ॥ कZ_०२.३०.१३ ॥
ata ūrdhvaṃ caturvarṣaḥ pañcavarṣo vā karmaṇyaḥ pūrṇa-pramāṇaḥ
From then onward, a four-year-old—or alternatively a five-year-old—is fit for service and is to be treated as of full standard measure (full-rated).
It builds administrative flexibility for differing breeds/conditions while still fixing a clear eligibility window for deployment and full rationing.