MBBS, BDS, BHMS, BAMS - admission to all these courses are done through NEET only. So in order to get into these courses, one needs to appear for Neet first. To be eligible for Neet, the candidate must be at least 17 years as on December 31 of the year of admission. And right now there is no upper age criteria but it's only until Supreme Court passes any judgement on it. So until and unless the Supreme Court makes any judgement on it, you can appear for Neet irrespective of your age. You can check the entire eligibility criteria here http://medicine.careers360.com/articles/neet-eligibility-criteria