Rule of thumb is: over 100 years and this is used by Customs to designate an antique on an imported item.  However,  the tariff act of 1930 designated anything made before 1830 as an antique;  so, I guess you could either say anything 100 years old or anything made before 1830 and make a good argument for you stance.   I think the tariff act of 1930 was establishing a hundred year age for an antique and not a cut-off date.