In a wireless device-to-device (D2D) network, mobile edge caching can reduce transmission cost for network traffic, but it may also cause outdated caching information. How to reduce the transmission cost and improve the freshness of information becomes an important issue. A dedicated caching device called helper which has a large cache can be used in a wireless D2D network, which can significantly improve the performance of network. In this paper, to better model the file-centric data transmission, we proposed a concept called age of file (AoF), which is defined as the duration from the latest updating of the file. We analyzed the AoF, energy cost and updating cost of the files in the network level. We comprehensively consider the AoF and energy cost through the maximum and minimum normalization methods, and proposed an AoF-based accessing strategy. In the strategy, users can adjust the access of files according to their demand for file freshness and improve the quality of service. The results indicate that this strategy can reduce the energy cost for file transmission and reduce the total cost of the entire network while satisfying the freshness of files.