Calculating duration between two dates in different rows based on duplicate value

This calculated column will do the trick:

Date Difference =
VAR CurrentAccount = [Account_ID]
VAR CurrentRecordSet =
    FILTER(
        'DateDiff Table',
        'DateDiff Table'[Account_ID] = CurrentAccount
    )
VAR AccountCount =
    COUNTX(
        CurrentRecordSet,
        'DateDiff Table'[Account_ID]
    )
VAR CreatedDate =
    IF(
        AccountCount > 1,
        MAXX(
            CurrentRecordSet,
            'DateDiff Table'[Created Date]
        ),
        0
    )
VAR SentDate =
    IF(
        AccountCount > 1,
        MINX(
            CurrentRecordSet,
            'DateDiff Table'[Sent Date]
        ),
        0
    )
VAR DateDifference =
    DATEDIFF(
        SentDate,
        CreatedDate,
        DAY
    )
RETURN
    DateDifference

It returns zero if there is only one record for an account.

If you want it to return the dates between sent and created even if there is only one record, get rid of the IF() function. So for example:

VAR SentDate =
    IF(
        AccountCount > 1,
        MINX(
            CurrentRecordSet,
            'DateDiff Table'[Sent Date]
        ),
        0
    )

becomes

VAR SentDate =
        MINX(
            CurrentRecordSet,
            'DateDiff Table'[Sent Date]
        )

Same logic for the CreatedDate variable. You didn't specify in your OP, so wasn't sure how you wanted those handled.