Work is forwarding and answering requests on the network. As you age you are in longer lasting groups. So the more age the more you have proven.
There is a debate whether nodes aged less than X ever need to get the data and respond to requests, but merely forward messages. This is not concluded yet, but it would mean that nodes not on the network for at least X chrn events could not influence the network until proven.
I like that idea as new nodes are like infants and should be allowed to go on and off while not upsetting the network adversely. New nodes will have done a resource_proof prior to joining. This means they must have "paid" more to join than the network has to "pay" to confirm them onto the network. An example can be found here https://github.com/dirvine/resource_proof
If you have rust installed then do
cargo install resource_proof and play around with that (
resource_proof -h). You can see it is a measure of cpu and bandwidth with the possibility of later also testing memory. The values should be network calculated but will probably be set by us initially.