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I'm asking in particular about this patent.
The filing year was 1991, so it is currently 23 years old. However, the fee status says "paid." Google Patents also does not mention anything about that patent being expired.
Is that patent expired, and if so, why does it not say so?
The term of a patent is 20 years from the filing date. The legal status of the patent will be "expired" in the case of a patent filed in 1991,. This is because the term of the patent has been more than 20 years.
Further, once a patent is granted, it is must for a patentee to pay the prescribed renewal fees to keep the patent in force until the completion of the patent term. The procedure and rules for payment of the renewal fees varies from jurisdiction to jurisdiction. It has to be noted that non payment of the renewal fees will lead to revocation of the patent.
http://www.invntree.com/blogs/can-lapsed-patent-be-restored-india
Further, the fee payment status is being shown as paid because renewal fees of the patent has been paid till the patent was in force.
This patent has expired.
There's no record because the USPTO doesn't change expired patents' statuses, but it definitely is. Since it was before 1995, it would either have expired after 17 or 20 years, and you're fine in either case.
The patent has already expired since it is more than 20 years. And once a patent term has ended it is not renewable. But just because a patent has expired doesn't mean it is already a public domain. Search if the owner has filed a continuation or improvement patent similar to the expired one. This is mostly done by inventors to extend their exclusive rights on the patent.
