How can I create calculation field in pivot?

[Anonymous]

I already have the cost of different products in different periods:

- Rows: Product types

- Columns: Years

- Measures: Cost of all products

Additionally I have a information about purchasing date of all products.

I want to see the cost development of each product in relation to its age. What exactly should I do to create a field so that one can see:

"product A, which had cost of €€€ in 2016, was 4 years old at that time" ?