I know that for doing Phd there is no age limit. If one will be interested to do a PhD after earning some money and finding a good topic for research will he be able do that.
What will be the amount of financial help will he be getting? How can it be done being on the job? And most of the universities offer financial assistance to international students. It depends upon the funds available with the department and the eligibility of the student.
You should get in touch with the concerned professor of the subject you are interested in the university you want to pursue your PhD.