In a previous study, we showed that visual spatiotemporal processing deteriorates with healthy ageing (Roinishvili et al, 2011, Vision Research, 51(4):417-23). Here, we used backward masking to investigate the underlying mechanisms. First, thirty-three older (M = 65.3 years) and ten younger adults (M = 24.3 years) had to indicate the offset direction of a vernier that was masked by a 5- or 25-element grating. We replicated results from our previous study with stronger masking effects in older compared to younger adults. Interestingly, stimulus onset asynchronies (SOAs) between target and mask were significantly longer in older adults with individually determined vernier durations above 30msec, compared to older adults with vernier durations similar to younger adults. Second, we used two different masks with either a spatial or a temporal inhomogeneity. Older adults with vernier durations that were similar to young controls were sensitive to temporal and spatial inhomogeneities in the mask indicating fast and spatially intact processing. However, older adults whose individually determined vernier durations were significantly longer than those of younger adults were not as sensitive to temporal and spatial inhomogeneities in the mask, indicating that small spatiotemporal details are filtered out, which is highly likely due to low-pass filtered vision.