Correct spelling, definition: twenties is a noun used to describe age, an era, or a temperature. It’s a combination of the numeral twenty and the suffix -s (changed to -ies). This word is spelled like that because of an important rule: when we want to create a plural form and there is a consonant before the letter -y, the suffix must be -ies, not -s. In this case, we have the consonant -t. Therefore, the only correct spelling is twenties, not twentys.

Definition of twenties:
plural noun, the numbers 20 through 29
The temperature must be in the low twenties today.
I’m in my early twenties, so I want to go to parties and have fun.

Collocations with twenties:
Some most commonly used collocations include: early twenties, late twenties, and to be in one’s twenties.
A young man in his early twenties wants to go out with me, so I’m going to say yes.
My mother had her first child in her late twenties.

Incorrect spelling, definition: when we want to create a plural from a noun whose last two letters are a consonant and -y, we need to change -y to -i and add -es. Twentys is an incorrect spelling since it omits this rule. The only correct form is twenties – it was made in accordance with the aforementioned principle.