You should be atleast 21 years of age and not more than 30 years. It has been mentioned in the official notification. You should be 21 years as on April 1st 2021. And if you will be 20 years and 8 months only by April 1st, 2021, you cannot apply for SBI PO 2021. You need to wait until  next year. There is upper age relaxation but there is no lower age limit relaxation. Every Candidate should be atleast 21 years as on April 1st of the year of recruitment.