Jeba,

• There is no upper age limit for NEET.
• And regarding 75% in 12th, if you have secured a minimum aggregate of 50% in Physics, Chemistry and Biology, you are eligible for NEET. It is 50% for General Category, 45% for PWD and 40% for OBC,SC,ST. Though you are a Post graduate, because the basic eligibility criteria for appearing in NEET is 12th, only 12th aggregate is considered. So in 12th in Physics, Chemistry and Biology your score should be 50%.
• If you have had atleast 2 subjects among Physics, Chemistry and Biology in your Bachelor's degree, your code will be 06. Code 06 says: B.Sc. Examination of an Indian University provided that he/she has passed the B.Sc. Examination with not less than two of the subjects Physics, Chemistry, Biology (Botany, Zoology)/Biotechnology and further that he/ she has passed the earlier qualifying examination with Physics, Chemistry, Biology and English. This is the most apt code for you.

For registration, use the link: https://ntaneet.nic.in/Ntaneet/Registration/NewReg.aspx . You can check the complete application process at our page, https://medicine.careers360.com/articles/neet-application-form.

But please do help us with the subjects you have studied in your B.Sc in the comments for the most relevant code.