Yes, you will be eligible for JEE Main and JEE Advanced in 2025 and 2026.

JEE Main allows candidates to attempt the exam in three consecutive years following their Class 12th examination. Since you are reappearing for the Class 12th board in 2024-25 due to medical reasons, your eligibility will start from 2025. This means you can take JEE Main in 2025, 2026, and 2027.

For JEE Advanced, candidates can appear in the exam twice in consecutive years, beginning the year they complete their Class 12th. Since your Class 12th completion year will be 2025, you can take JEE Advanced in 2025 and 2026.

Your gap year, caused by health challenges, does not affect your eligibility. The examination authorities recognize that candidates might face unforeseen circumstances and determine eligibility based on the year of completion of Class 12th, not initial enrollment. Just ensure you meet the other basic requirements, such as minimum marks in the required subjects, when applying.

Focus on your preparation and health, and when you're ready, you’ll have the opportunity to pursue JEE with full eligibility.