There are a few methods:
1) In Customers, create a calculation (result is date) with: Max ( OrderDate )
2) If the Order date is the creation date (or at least always the latest order is the most current), then you can also use calculation of Last (OrderDate).
3) Create another table occurrence of your orders table and attach it to your Customer table by CustomerID. Sort the relationship in descending order of Order Date then simply place your Orders::Order date directly on your Customer layout.
4) Create a one-row portal of your base relationship to Orders on your Customer layout. Only place Orders::OrderDate within it. Make portal small enough to only hold that order date. Sort the portal in descending order on OrderDate.
I would suggest item 4. In all instances, be sure to disallow entry so Users can't accidently change that OrderDate.