This happens until age of 30 days, again, with the assumption that these are large amounts. Sometimes, a small amount might stake between 7 and 30 days.
Amounts older than 30 days will be grouped, up to 100 DMD to increase their chance to stake.
With increasing coin age the possibility to stake successful is increasing so even for people with very low amount of DMD will be able sooner or later earn their POS rewards.
There is no upper limit on coin age that can be staked.