Hello,

20 years age is the ideal age. You prepare now. After graduation, you can appear for bank exam. There won't be any issue. You can appear for it. There won't be any issue.

There are candidates whose graduation complete at 22-23 age and appear for bank exam.  So it is okay if you appear after your graduation with full preparation.

If you are weak in English ability then take help of book examples. Watch videos available in Youtube where they teach you tricks and rules for easy solving. It can be hard now but if you practice daily then it will get easy for you no matter from which board you have passed.

Hope it helps!