The word an is used for ordinal numbers (1, 2, 3, etc) , unless an adjective is used to describe the word year. In that case the word années is used. Sometimes either word is acceptable.

adjective: "a word naming an attribute of a noun, such as sweet, red, or technical."

It depends on how you view the year, where if it is a point in time, as in using an ordinal number, for the purposes of counting or stating a date, then use the masculine l'an

Use the feminine année is it is :
- more subjective, based on personal feelings, tastes or opinions,
- Focusing on the elapsed time and events that occur in a particular year.

eg.

• I am 20 years old : J'ai vingt ans.
• I ate there three years ago : J'ai mangé il y a trois ans.
• I was a student for four years. : J'étais étudiant pendant quatre ans.
• It was in the year 2000. : C'était en l'an 2000.

• In the year 2020 : En l'an 2020
  counting years :

• every single year : tous les ans

• every other year, every two years : tous les deux ans

While for experiencing events and with quantifiers of few, many, and indefinite counting, as well as adjectives, you use année

• Last year was good : L'année dernière était bonne
• Two good years : deux bonnes années
• It was some years ago. : C'était il y a quelques années.
• In the third year. : Dans la troisième année.

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As always , if I have anything incorrect, I trust someone will point this out. I am also learning French, and not a master of it. ༠