Two different questions get conflated under "NEET eligibility," and only one of them has a stable answer.
What's stable: the academic requirement
To sit NEET, a candidate needs to have passed (or be appearing for) Class 12 (or equivalent) with Physics, Chemistry, Biology/Biotechnology and English as subjects, from a recognised board. This structural requirement — the four subjects, the recognised-board condition — has been consistent for years and is the safest thing to plan around.
Separately, admission to an MBBS/BDS seat (once you've qualified NEET) requires a minimum aggregate in PCB in that qualifying exam, which is lower for reserved categories than for general — this is a different number from the NEET score percentile that qualifies the exam itself (covered on the cut-offs page), and the two get confused constantly. One is a Class 12 marks requirement to be admission-eligible at all; the other is where your NEET score has to land relative to everyone else's that year to clear the exam.
What's not settled: the age limit
The upper age limit for NEET has been removed, reinstated, and litigated more than once by the NMC and in court. Because of that history, this article deliberately does not state a specific number — any figure printed here could be the version that's currently overturned by the time you read it. Check the eligibility section of the current NTA information bulletin directly before assuming you either do or don't qualify on age grounds; this is the one part of NEET eligibility where "what everyone says" and "what's currently in force" have diverged before.
The lower age bound — a minimum age by a fixed date in the admission year — has been more stable, but confirm the exact date and figure against the same bulletin rather than a summary (including this one).
Attempts
There is currently no cap on the number of times a candidate may sit NEET. This has also been debated in the past (a proposed cap was floated and not implemented), so it's worth the same caveat: confirm against the current bulletin if this is a factor in your own planning, rather than assuming past-years' rules still hold.
What to do with this
None of the above changes what you actually need to study — the syllabus and pattern are the stable parts of NEET, and they're what this site is built around. Eligibility questions are a compliance check to make once, against the primary source, not a moving target to prepare for.
This article intentionally omits specific age-limit and minimum-percentage figures because they have changed and been litigated in the recent past. Read the current official NTA information bulletin for the figures that apply to your own attempt — this is the one area of NEET eligibility where a summary, including this one, can be confidently wrong.