As you are 20 now and the required age is 21 so for now you cannot apply for SBI exam. Now you have to wait for 1 year and so that for this time you can prepare more better for your 1st attempt.
As per the eligibility criteria candidate should be 21 years old and not more than 30 years old at the time of registration. So , for 2020 you are not eligible you can give the exam in 2021.
Also , there are some age relaxation as per government rules for different categories like for Scheduled Caste/Scheduled Tribe (SC/ST) age relaxation is 5 years.
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