Federal Rule:
The IRS allows a taxpayer to file an amended return for the three prior years. Please view the TurboTax FAQ below for more information and filing instructions.
https://ttlc.intuit.com/replies/3288565
Maryland Rule:
Generally, Form 502X to file an amended Maryland tax return must be filed within three years from the date the original return was due (including extensions) or filed. However, there are exceptions to this rule. Please view page 5 of the Maryland link below for more information.
http://forms.marylandtaxes.gov/current_forms/502X.pdf