The age of index in Data Stream with ILM policy

hello, I'm confuzed about something. it seems like ILM policy only manage indices, not data stream.
For example, There is a data stream and index . the current write-index of data stream is .ds-e-000001, it's age is 5d.
when rollover happens, it will create new index .ds-e-000002, change the current write-index of data stream to new index, and the age of these two index will refresh, make it 0. it seems like rollover operation make the time attribution of index recount ,but why?
And I find something about index.lifecycle.origination_date , here is link .