Suppose my birthday was in August and now I am 20 as now it's September, before August my age was 19. Can I say "I am at my early 20."?
Suppose my birthday is in October and now I am 20, after October I will turn to 21 so I am very closer to 21 but not yet. Can I say "I am almost 21."? I think it's okay. Another one is can I say "I am at my late 20."?
Any help please.
You can use in my early/late twenties.
This is not an exact "rule" but:
I am 20 = I am exactly 20 years old.
I am in my early twenties = I am 21/22/23 years old.
I am in my mid-twenties = I am 24/25/26/27 years old.
I am in my late twenties = I am 28/29.
I am 30 = I am no longer in my twenties.
Note that "mid-twenties" requires a hyphen, but "early/late twenties" do not.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.