German here. Just checked my car seats, and there is no expiration date on them.
However, we have (European) safety norms that are updated every few years. Seats with a certification before 1995 can no longer be sold or used. The pre 2005 models are not recommended but still pop up on Ebay.
To actually add an answer: here, seats „expire“ because they no longer meet the updated safety norms.
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