There is no such tag in FTM.

You can:

1) enter an estimated birth date, which will be used in privatization procedures (either within FTM or ancestry.com or worldconnect.com or wherever) by the criteria of: privatize people under x years old that don't have a death entry.

2) Enter anything in the Death Place field (eg USA), or certain text strings in the Death Date field: "?", dead, deceased, infant, etc - see the Help file for the complete list.