Car seats have expiration dates because the properties of plastics and foams change over time. They are less able to hold up (predictably, reliably) during impact as they get older. I would use a car seat past 5 years if I purchased it new from a reputable source, it had been in our family the entire time (not loaned to someone), it was never in any level of accident, and it was stored appropriately when not in use. Under those circumstances, I'd be comfortable using a car seat for 6 or 7 years, though I'm no professional (ie don't do what I do without discussing it with someone with more car seat reliability background). That is just something that everyone has to decide on their own- you are the one who'd have to live with the consequences of something going wrong.
Back to your post: since your play pen doesn't have to withstand a car crash, there are totally different safety standards. Check it over- make sure none of the plastic parts are cracked, no holes in the mesh, no hanging straps or threads, everything is in good working order. Everything in our nursery was bought used from craigslist, and already went through one or both kids. I had to tighten/replace some screws and bolts, but everything looks good for another go-round. The biggest risk you run in this situation is if play pen safety standards have changed significantly in the past 10 years. I bet you can find some of that info with a google search.
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