Check the expiration date. It should be stamped onto the plastic, or on a sticker on the side. If there is no sticker and you don't know the date, don't risk it. The plastic is high grade, but its still only plastic and it does break down and get weak. Its not worth giving it away or putting it on freecycle, because you are putting someone elses child in danger.
THe plastic can be recycled, take it to a center and let them take it apart. If you do choose to toss it, remove the cover, remove the harness, and write' DO NOTUSE' in black marker on the plastic so nobody dumpster dives for it.
car seats expire after 6yrs. THe plastic, while higher grade, is still just plastic and goes through the same photodegeneration that any plastic does. Just like that faded plastic toy in your back yard, plastic in a car seat is subjected to heat and cold and wear, and it DOES break down. Even if you can't see it, why risk it? The expiration dates are there for a reason, because thats when the plastic failed in testing.