Is it "x years old" or "x years' old"?

[BecauseImWorthIt]

I think it's the latter, but seeing it written down it looks all wrong!

No apostrophe.

I would say x years old.

I think the former.

the former - it's not possessive.

The x years doesn't own the old.

no apostrophe as it's a plural ie the number of years.

I think I was getting mixed up with "years of age", and "days' worth"

Thought it looked wrong!

Thank you all. I hate getting abbreviation wrong!

years