Select the relevant population

You want to construct a predictive model that selects donors that are most likely to donate to a campaign in June 2018. You want to give these donors a call on June 1st 2018. For this specific campaign, you want to work with young donors only, i.e. age 18 to 25 (inclusive).

To build the model, you construct a basetable with a timeline that is exactly one year earlier than the true timeline.

What age should the donors in the population of the basetable have?