In principle, any document that is state of the art under Art. 54(2) EPC may be a candidate for the closest prior art. The jurisprudence acknowledges, however, some cases where a document may not be a realistic starting point because it either relates to outdated technology, and/or is associated with such well known disadvantages that the skilled person would not even consider trying to improve on it. However, a document's age as such is not a ground for excluding it as closest prior art and as a starting point for the assessment of inventive step (T 1408/04).

In T 334/92 the board held that a document that had been disregarded for more than 20 years by those skilled in the art, had never been used during that period as a basis for further development, was moreover completely silent about the extent of the indicated activity and, finally, did not even mention, let alone discuss, the relevant state of the art, did not represent the closest state of the art and could not therefore be used to define a realistic technical problem. However, in T 964/92, filed as a divisional application to T 334/92, the board held that the same document could be considered as a realistic starting point for the determination of the relevant technical problem.

In T 1000/92 the board did not agree to the selection of document (1) as the closest state of the art because the disadvantages of the process described in document (1), which had been published about 30 years before the priority date of the application, were so evident and well-known that a skilled person would not have tried to improve and develop such an old process (see also T 616/93). In T 823/03 the board stated that the skilled person had good reasons not to select D2 content as a basis for further development since too many uncertainties existed.

In T 479/00 the board did not regard a 65-year-old document as a realistic starting point for the evaluation of inventive step. It was unrealistic to assume that, without hindsight, somebody of average skill in the art of colouring ceramic articles in 1994 would have had the intention to improve a technique which had not received any attention during the previous 65 years. Furthermore, the teaching of this document, published in 1929, had never been put into practice on a commercial scale.

However, in T 69/94 the board pointed out that Art. 54(2) EPC 1973 defined the state of the art as comprising everything made available to the public, clearly without any time restriction. Thus a document which related to an antiquated technology no longer used in industry comprising a teaching disapproved by those skilled in the art at the filing date of the patent in suit could not simply be disregarded as the closest prior art only because of its publication date about 20 years before the filing date of the application documents. In T 113/00 the board said that resurrecting very old teaching (in the case in hand 31 years old) with an obvious modification did not make known subject-matter inventive. In T 1397/07 the board added that there was no legal basis in the EPC to preclude a particular state of the art from being taken into account when assessing inventive step, merely because of being published some years earlier than another one. In T 153/97 it was also stated that there was no plausible reason why the skilled person should have disregarded a document only because the publication date lay 30 years in the past.