How 'old' are the documents?

Hello,

I have need to create a plot that shows how long the documents exists in the index
My first idea would be creating a scripted field that computes something like now()-timestamp
How can this be performed in kibana?

Yes, your intuition is good.
This is how to calculate the difference between 2 dates in Kibana scripted fields: Kibana, getting todays date

There will be small differences in seconds based on how many documents the calculation is done for, but I guess you don't need that kind of precision.