Two structural-diversity and two biodiversity indexes and carbon stocks were determined at eight humid and sub-humid tropical forest sites 20–120 years old in southeastern Mexico. All trees >10 cm at diameter breast height were measured and species identified in 131 randomly located plots. The number of species (S), Shannon (H′), biomass-species (BS) and abundance-biomass-species (ABS) indexes were related to Mg C ha^-1 and the age of the sites. Values were: S, 27–109; H′, 3.9–5.31; BS, 7.39–16.0; ABS, 9.87–18.2; Mg C ha^-1, 39.7–130.7. The relationships between BS and ABS to carbon and the site age were linear and positive, showing that stand structure directly influenced carbon storage. The relationship between site age and carbon was remarkably high (R² = 0.79). Humid forests showed higher values of carbon than sub-humid forests (104.4 vs. 70.6, t = 2.07, p = 0.04) but their structural diversity was similar (BS, 11.25 vs. 8.3, p = 0.134; ABS, 13.9 vs. 10.6, p = 0.135). It is concluded that forestry management of tree diameter may be a factor that can favor more carbon storage in secondary forest regrowth as it starts to reach maturity.