Forest cover has consistently increased since 1850 in Western Europe. This afforestation mainly occurred on abandoned agricultural land. It allows distinguishing between ancient forests – in continuous existence for centuries – and recent forests. Forecasting the response of biodiversity to forest continuity and its interaction with stand maturity may help to better achieve the ecological restoration of forests and prioritize conservation strategies. Saproxylic beetles are a species-rich group that plays functionally important roles in forest ecosystems. Controlling for the effect of fragmentation, we developed a balanced sampling design that crossed forest continuity by stand maturity in montane beech-fir forests. Then, we established 40 plots where we sampled saproxylic beetles using flight-interception traps and Winkler-Berlese extractors. Overall, 307 saproxylic beetle species were identified. Contrary to the hypothesis, species diversity and composition was not influenced by forest continuity. Instead, in both ancient and recent forests, saproxylic beetles were positively influenced by stand maturity. At the species level, though more species were associated with stand maturity than with forest continuity, even more species were associated with the interaction between forest continuity and stand maturity. The interaction effect is confirmed when accounting for morphological (body size) and ecological traits (diameter, decay stage and canopy cover niche position) of saproxylic beetles: overmature stands in ancient forests benefited to larger species and to species depending on large deadwood pieces. In unfragmented forests, as long as stand attributes are of suitable quality, saproxylic beetles may colonize newly available habitats. However, the functional structure of saproxylic beetle assemblages was best preserved in ancient forests, especially in overmature stands. This indicates that (1) forest continuity act congruently with stand maturity in providing valuable habitat resources for biodiversity and that (2) overmature stands in ancient forests are of greater value for conservation than overmature stands in recent forests.