Most students start studying for NEET/ JEE exam after their 10th exams. Most students join coaching institutes for their Neet/JEE and Board exams.
In some cases students also start preparing for NEET/JEE exam when they are in 6th/8th/9th or 10th standard.
Some students do self study/school for 2 years after their 10th class exams and take 12th board exams, they then take a drop year and prepare for the exam in a coaching institute.
It does not matter that when you start preparing for exam, what matters is your consistency, hard work and passion for your exam.