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Interstellar dark clouds are the sites of star formation. Their maincomponent, dihydrogen, exists under two states, ortho and para. H2 is supposedto form in the ortho:para ratio (OPR) of 3:1 and to subsequently decay toalmost pure para-H2 (OPR <= 0.001). Only if the H2 OPR is low enough, willdeuteration enrichment, as observed in the cores of these clouds, be efficient.The second condition for strong deuteration enrichment is the localdisappearance of CO, which freezes out onto grains in the core formationprocess. We show that this latter condition does not apply to DCO+, which,therefore, should be present all over the cloud. We find that an OPR >= 0.1 isnecessary to prevent DCO+ large-scale apparition. We conclude that theinevitable decay of ortho-H2 sets an upper limit of ~6 million years to the ageof starless molecular clouds under usual conditions.

Ortho-H2 and the Age of Interstellar Dark Clouds