Hello,
as you know, the OPC Server can provide values from Device, or from Cache. We (in the EasyOPC component) add one more source, from our own "cache" inside the component - we call "from Memory",
Let's say that at time t, we (in the component, for whatever reason), obtain a value from the OPC Server from the Device. And, let's say that DesiredValueAge = 1000 (ms).
If you call EasyDAClient.ReadItem with this (and Source-ByValueAge)

at t+500 ms, we will not contact the OPC Server at all, and give you value from Memory
at t+1500 ms, if the server support OPC-DA 3.0 and therefore the MaximumAge argument to Read, we will call the OPC Read with MaximumAge=1000, and the server will decide whether to provide from Cache or Device. If the server only supports OPC-DA 1.0 or 2.0, we will call OPC Read from Device.

There some more complicated scenarios as well, for example if we also have a subscription, but the bottom line is that you will get the value that is not older (in terms of obtaining from Device) than DesiredValueAge, if Source=ByValueAge.
Best regards,