Purchasing bitters underage?

I’m 20 and want to make some mocktails and I recently tried to buy some bitters but got carded. I thought that bitters were classified as non-alcoholic in the same way that vanilla extract is, despite vanilla extract being 35% abv. Is this true, or did the cashier just not know what they were talking about, and could I buy it through the self checkout lane to avoid the confusion?