As of update 14 (2023-01-18) of the Standard Content we added the properties “minimumDate” and “maximumDate” to the DatePicker and DateTimePicker (OpenUI5 1.96).
Both properties are strings that are converted to Javascript date objects internally to set the UI5 standard properties minDate and maxDate.
If you want to allow only dates larger than 18 years you can dynamically calculate the string for the correct date 18 years ago (or in the future) in a Business Object and use the output to set the min/max property of your Picker.