Kautilya's Arthashastra 2.31.11

2.31.11

saptāratni utsedho navāyāmo daśa pariṇāhaḥ pramāṇataś catvāriṃśadvarṣo bhavaty uttamaḥ, triṃśadvarṣo madhyamaḥ, pañcaviṃśativarṣo 'varaḥ //

(That which is) seven aratnis in height, nine aratnis in length, ten aratnis in circumference and is (as can be inferred from such measurement), 40 years old, is the best.

That which is 30 years old is of middle class; and that which is 25 years old is of the lowest class.