It is really hard to date pieces from overseas - especially if there is no marking or documentation. In the US they generally mark in an inconspicuous way to prove compliance to the law. (Even dated photos with items can help to document items.)
Just like any antique....it has to be 100 years or more of age in order to be an antique. In order to comply with US law the item has to have been imported before
More information is noted in the Protected Species Act, Section 10 (h)
"(h) CERTAIN ANTIQUE ARTICLES.-
(1) Sections 4(d), 9(a), and 9(c) do not apply to any article which-
(A) is not less than 100 years of age;
(B) is composed in whole or in part of any endangered species or threatened species
listed under section 4;
(C) has not been repaired or modified with any Dart of any such species on or after
the date of the enactment of this Act; and
(D) is entered at a port designated under paragraph (3).
(2) Any person who wishes to import an article under the exception provided by this subsection
shall submit to the customs officer concerned at the time of entry of the article such
documentation as the Secretary of the Treasury, after consultation with the Secretary of the
Interior, shall by regulation require as being necessary to establish that the article meets the
requirements set forth in paragraph (1) (A), (B), and (C).
(3) The Secretary of the Treasury, after consultation with the Secretary of the Interior, shall
designate one port within each customs region at which articles described in paragraph (1) (A),
(B), and (C) must be entered into the customs territory of the United States.
(4) Any person who imported, after December 27, 1973, and on or before the date of the
enactment of the Endangered Species Act Amendments of 1978, any article described in
paragraph (1) which-
(A) was not repaired or modified after the date of importation with any part of any
endangered species or threatened species listed under section 4;
(B) was forfeited to the United States before such date of the enactment, or is subject
to forfeiture to the United States on such date of enactment, pursuant to the
assessment of a civil penalty under section 11; and
(C) is in the custody of the United States on such date of enactment; may, before the
close of the one-year period beginning on such date of enactment make application to
the Secretary for return of the article. Application shall be made in such form and
manner, and contain such documentation, as the Secretary prescribes. If on the basis
of any such application which is timely filed, the Secretary is satisfied that the
requirements of this paragraph are met with respect to the article concerned, the
Secretary shall return the article to the applicant and the importation of such article
shall, on and after the date of return, be deemed to be a lawful importation under this
Act."
