Little is known about the potential of restoration plantations to provide appropriate understory conditions to support the establishment of seeds arriving from neighboring native forests. In this article, we investigated how seedling establishment is affected in the understory of restoration sites of different ages and assessed some of the potential environmental factors controlling this ecological process. We first compared the density and richness of native tree seedlings among 10-, 22-, and 55-year-old restoration plantations within the Atlantic Forest region of southeastern Brazil. Then, we undertook a seed addition experiment in each study site, during the wet season, and compared seedling emergence, survival, and biomass on local versus old-growth forest soil (transferred from a reference ecosystem), in order to test whether local substrate could hamper seedling establishment. As expected, the oldest restoration site had higher density and richness of spontaneously regenerating seedlings. However, seedling establishment was less successful both in the oldest restoration planting and using substrate transferred from a reference ecosystem, where emergence and survival were lower, but surviving seedlings grew better. We attribute these results to lower light availability for seedlings in the understory of the oldest site and speculate that higher incidence of pathogens on old-growth forest soil may have increased seedling mortality. We conclude that the understory of young restoration plantations provides suitable microsite conditions at the early establishment phases for the spontaneous regeneration or enrichment planting of native trees.