2-year, 2 years' or 2 years

上面o個位朋友就o岩喇
only two-year experience is correct
two-year old
i have a two-year experience in XXX
i have two years of experience in XXX

ex: You should have at least two years of relevant work experience
ex: you should have six years' experience with two to three years in system analysis and design. ...
ex :A two-year experience.

Out of the three options, only the first one is correct. When you use a hyphenated representation to create an adjective, you *always* use the singular form of the noun.

My two-year experience in my previous position has helped me secure the offer.

You can also say "two years of experience":

My two years of experience in my previous position...

Either 2 years experience or 2-year experience is also correct.

As 2 is a plural form, so year should be with s. However, if a hypen (-) puts between 2 and year, two of them formed a word, so it's a single form which year should not be followed by s.

2007-06-01 10:28:18 補充：
But 2 years experience is a more appropriate form.