A good measure to go by, at least in the United States, is 50 years old. This is usually used when looking at structures or sites, but can also be extended to artifacts.

The term antique really has no use in any form of historical research. The term artifact is usually given to an object that has some relation to a specific location/phase/etc. So an old lamp in your attic may not be an artifact specifically, but perhaps it has some historic significance and that could make it an artifact of some sort. It's really a kind of grey area, it depends on heavily on what the item is that you and where it's from. If you have an old bottle in your cupboard it may just be an interesting piece, but if that same bottle was found in an excavation of an early 20th century structure, then it may be an artifact vital to dating the site.