The grass season is a bit odd in a way- it feels like a long time since we have seen any of them play on gras- and a lot of the younger players really don't have much of 'past' on the surface. It will be interesting to see both how they do- and how the 'older' players do, when they come up against players they might not have played much on grass before.
Federer, in particular, has been finding that in his absence, the younger players have been getting a lot better.