Can you file 2018 state and federal taxes using the free edition online? If yes, how do you do it?

I'm sorry, but no.  The Online platform is only for current-year returns.  In order to file a prior-year return, you'll need to purchase, download and install the TurboTax software for that year.  Returns for prior-year must be printed and mailed.  They cannot be e-filed.  See the info linked below for more instructions.

How do I prepare a prior-year tax return? 

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NO.   Online is only for 2019 returns.  You cannot prepare a 2018 return online.

Online preparation and e-filing for 2018 is permanently closed.

To file a return for a prior tax year

If you need to prepare a return for 2016, 2017, or 2018 you can purchase and download desktop software to do it, then print, sign,  and mail the return(s)

https://turbotax.intuit.com/personal-taxes/past-years-products/

Remember to prepare your state return as well—if you live in a state that has a state income tax.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Note:  The desktop software you need to prepare the prior year return must be installed/downloaded to a full PC or Mac.  It cannot be used on a mobile device.

If you are getting a refund, there is not a penalty for filing past the deadline.  If you owe taxes, the interest/penalties will be calculated by the IRS based on how much you owe and when they receive your return and payment. The IRS will bill you for this; it will not be calculated by TurboTax.

https://ttlc.intuit.com/questions/1900990-how-do-i-file-a-prior-year-tax-return

To prepare 2018 for free:

2018 Form 1040, Schedules and Instructions

https://www.irs.gov/pub/irs-prior/f1040--2018.pdf
 https://www.irs.gov/pub/irs-prior/i1040gi--2018.pdf
2018 Tax and EIC Tables https://www.irs.gov/pub/irs-prior/i1040tt--2018.pdf

IRS 2018 Schedules 1 thru 6 (only if needed)
Sch 1 : https://www.irs.gov/pub/irs-prior/f1040s1--2018.pdf
Sch 2 : https://www.irs.gov/pub/irs-prior/f1040s2--2018.pdf
Sch 3 : https://www.irs.gov/pub/irs-prior/f1040s3--2018.pdf
Sch 4 : https://www.irs.gov/pub/irs-prior/f1040s4--2018.pdf
Sch 5 : https://www.irs.gov/pub/irs-prior/f1040s5--2018.pdf
Sch 6 : https://www.irs.gov/pub/irs-prior/f1040s6--2018.pdf

You would have to get the state forms from the state web site.

https://ttlc.intuit.com/questions/1901670-how-do-i-contact-my-state-department-of-revenue

No, there is a charge. 

Prior Year TurboTax software