Learn English – “5 years experience in” or “5 year experience in” or “5 years of experience in”

Here are some facts would make it more clearer:

Years are countable so using 5 years would be correct.

Oxford dictionary says:

[countable, usually plural] age; time of life

-He was 14 years old when it happened.

-She looks young for her years.

-They were both only 20 years of age.

-A twenty-year-old manHe died in his sixtieth year.

-She's getting on in years (= is no longer young).

Experience can be used as both countable and uncountable

Countable forms:

[countable] an event or activity that affects you in some way

-an enjoyable/exciting/unusual/unforgettable, etc. experience

experience (of something):

-It was her first experience of living alone.

Uncountable forms:

[uncountable] the knowledge and skill that you have gained through doing something for a period of time; the process of gaining this

-to have more than ten years of teaching experience

-Do you have any previous experience with this type of work?

also

[uncountable] the things that have happened to you that influence the way you think and behave

-Experience has taught me that life can be very unfair.

-It is important to try and learn from experience.

So here we are looking for the first uncountable form which fits exactly what we are looking for.

Which gives us the result:

-I have 5 years of experience in playing chess.