Hello Ezra,
I am unable to recreate this same issue. Are you still experiencing this?
Here are the steps I'm going through, maybe there is something different we are doing:
1. Under Account Schedules, I selected M-BALANCE which is my Balance Sheet report that is selected in General Ledger Setup. When I print from the Account Schedule I get the following options:

2. When I'm on my home page and select Financial Statements from the drop down and click Balance Sheet, my starting date is grayed out just the same:

When you go into your General Ledger Setup window and click the Reporting Fast Tab, is the appropriate Account Schedule selected for the Balance Sheet report?
Regards,
Jennifer Reinhardt
Please see our Dynamics 365 Financials Support Team Blog for helpful information:
https://blogs.msdn.microsoft.com/dyn365finsupport/