When can we start filing taxes for tax year 2018 (filed in 2019)

There is absolutely no reason to file an early tax return for 2018.

Filing a tax year 2018 tax return will start after January 1, 2019.  However, the IRS will probably not start to accept any 2018 tax return until late January 2019.  Due to the tax code changes and the changes to the federal tax forms it would probably be wise to wait until mid to late February to file the 2018 tax return.

The TurboTax software for tax year 2018 will not be available until sometime in either November or December 2018.  And the software will need several updates before it can be used to file a tax return.  TurboTax has to wait for the IRS to finalize the federal forms and schedules and then must have IRS approval for the forms and schedules included in the TurboTax software.

Addendum 01/04/2019 -

As of this date - The IRS has yet to publish a date in 2019 when they will be accepting any type of  2018 tax return.  It is estimated that the IRS will probably not start to accept any 2018 tax return until late January or early February at the earliest.  Due to the tax code changes, the changes to the federal tax forms and the partial government shut-down, it would probably be wise to wait until mid to late February to file the 2018 tax return.

You cannot file your tax return using TurboTax until all the forms and schedules included in your tax return have been finalized.  Click on the Review tab at the top of the program screen.  The forms and schedules will be listed with the estimated date for being available.

Addendum 01/08/2019 -

The IRS has announced that they will be accepting 2018 tax returns starting on January 28, 2019.

IRS announcement - https://www.irs.gov/newsroom/irs-confirms-tax-filing-season-to-begin-january-28

If your tax return has a tax refund which includes either the Earned Income Tax Credit (EITC) and/or the Additional Child Tax Credit (ACTC) the IRS is required to hold the tax refund until mid-February due to the PATH Act passed by Congress.  The IRS will release the refund payments during the latter part of February.