Incorrect spelling, explanation: twentys is a word that is almost correct – but almost doesn’t mean entirely. The general idea is there, there is the base word twenty and a suffix s to make a plural noun. But it’s important to remember, that y at the end of a word doesn’t change in plural form only when there’s a vowel before (like in boys it’s an o). In the case of twenties, the letter before last is t, so the y has to become ie.

Correct spelling, explanation: twenties is plural for twenty, so the word formation process it undergoes is pretty straightforward: we take twenty and add a plural suffix s. In this case, however, the y has to be changed to ie in twenties, because of the rule that when there’s a consonant before the last letter, the y has to be changed to ie.

Definition of twenties:
noun
1. the general term for numbers 20 and 30, is often used in the context of age, era, or temperature.
He looked young, he was probably in his early twenties.
The attendance was lower than we expected, and the number of guests did not surpass the twenties.