Hi spl_ind,
Technically and by rule items over 100 years aged are considered as antique and its mandatory to register them with government moreover you should also notify change of ownership and/or form to goverment. Its strictly prohibited to export such items without permission of ASI.
But as the word suggest anything old can be considered as antique normally 15 to 20 years is enough timeframe to consider your item as antique
With Regards
Rishikesh