The age discrimination act can be found
here.
I cannot see where it would be unlawful for a foreign (or UK) company soliciting employees for employment within a foreign jurisdiction, to advertise for age (or other) restricted vacancies. For example if you consider Bae looking for employees to work in Saudi Arabia, you would find the following stipulation in their advertisement.
BAE Systems Saudi Engineers
Please note that we are committed to equal opportunities and diversity in our workforce, but also respect the culture, beliefs, legal and customer requirements in the countries in which we operate. Employment in Saudi Arabia is subject to the Company's ability to obtain a visa for work in the Kingdom and therefore we are only inviting male applicants for these roles.
Extract from The Employment Equality (Age) Regulations 2006.
Meaning of employment and contract work at establishment in Great Britain
10. —(1) For the purposes of this Part ("the relevant purposes"), employment is to be regarded as being at an establishment in Great Britain if the employee—
(a) does his work wholly or partly in Great Britain; or
(b) does his work wholly outside Great Britain and paragraph (2) applies.
(2) This paragraph applies if—
(a) the employer has a place of business at an establishment in Great Britain;
(b) the work is for the purposes of the business carried on at that establishment; and
(c) the employee is ordinarily resident in Great Britain—
(i) at the time when he applies for or is offered the employment, or
(ii) at any time during the course of the employment.
(3) The reference to "employment" in paragraph (1) includes—
(a) employment on board a ship only if the ship is registered at a port of registry in Great Britain, and
(b) employment on an aircraft or hovercraft only if the aircraft or hovercraft is registered in the United Kingdom and operated by a person who has his principal place of business, or is ordinarily resident, in Great Britain.