You can roll a lawn at any stage. Generally grass is rolled soon after sowing to improve the contact between seed and soil, improving germination. However, rolling at a later stage may still be a good idea to push down any small stones that may be on the surface. The main thing is not to roll it when the ground is very wet/soft as this will only compact it and lead to poor drainage. On the other hand, if the ground is very dry and rock hard there is also no point, somewhere in between is best.