Ph.D

Yes you can, there is no age limit for pursuing Ph.D.

Hi Aman,
To apply for Ph.D. programs you must write to the universities, the department supervisors precisely. You must share your research ideas or proposal, they will accept you only if they find your research thoughts interesting. Research on the universities, check what all universities actually

Definitely not, they can offer consultancy and entrepreneurial services as well as apply for suitable positions in public and private sector companies.

Yes, you can pursue integrated PhD in biological sciences after completing BDS. Kindly refer to the link below:
http://www.winentrance.com/admission_notification/tata-institute-fundamental-research-admission.html

You have to qualify the entrance exam of the university for pursuing Ph.D.