Is there a way to customize the date of birth block to be the American style of the order of Month, Day, Year instead of Day, Month, Year?
The display/layout order of widgets in a block can be modified using the 'Layout Assistant' tab of the block.
On this tab select each widget in the left panel one at a time and set their 'Layout Order'. To achieve what you're looking for, set the following:
Day - Layout Order: 2
Month - Layout Order: 1
Year- Layout Order: 3
Save and Preview your form to see the change.