I think that a creation timestamp would be very useful and it does not cost very much. It would be an immutable value.
Nowadays (on 4.3.1) I must keep that info together with the id of the ledger, for instance in zookeeper, but if that reference gets lost there is now way to know how old a ledger is.
I think that this timestamp should be captured on client while calling createLedger or asyncCreateLedger.
In addition to this very common field maybe it would be useful to add a custom byte field named "custom client data" in order to let the client 'describe' the ledger without the need of extra data on Zookeeper.