Identifying premature expiration - sql

Question

The dataset I have is a bit tricky. It’s a rolling calendar for a period of 24 months, and the data is published only once a month.
The relevant data points are as follows:
• CaseNumber (int)
• Start_date (date)
• Reporting_month (date)
• Months_old (int)
The above ‘CaseNumber’ has the ‘potential’ to appear in a ‘Reporting_month’ as many as 24 times (0-23). However each ‘CaseNumber’ will only appear one time in each ‘Reporting_month’.
So if you list any # of months in chronological order (Jan, Feb, Mar, Apr, etc.) a single ‘CaseNumber’ is will show up in each one of those ‘Reporting_months’ as long as the ‘CaseNumber’ is < 23 ‘Months_old’.
However, once a ‘CaseNumber’ = 24 ‘Months_old’ it will no longer report in this data set. So the oldest any particular ‘CaseNumber’ will ever be in this reporting cycle is 23 ‘Months_old’. Any older, than it will not appear on this report.
What I’m interested doing is tracking these ‘CaseNumbers’ to see if any are dropping off of this report prematurely. So in doing so I need to be able to compare the current ‘Reporting_month’ to the previous ‘Reporting_month’ to determine if any of the ‘CaseNumbers’ prematurely dropped off.
Example:
Case # Previous Current
Months_old Months_old Status
1234 22 23 Correct age
5678 23 NULL Dropped due to age
9101 18 NULL Premature drop
only means i've been able to achieve this is via a VLOOKUP formula in excel done manually. I'd like to get away from having to complete this manually.
SELECT
a.[CaseNumber]
,CONVERT(DATE,MAX(a.[Month]),111) 'Month'
,CASE WHEN m2.[CaseNumber] IS NOT NULL
AND m1.[CaseNumber] IS NULL
THEN 'Yes'
ELSE 'No'
END as 'New Default'
FROM
[dbo].['v2-2yrTotalDefault$'] a
LEFT OUTER JOIN (
SELECT DISTINCT
[CaseNumber]
FROM
[dbo].['v2-2yrTotalDefault$']
WHERE
LEFT(CONVERT(varchar,[Month],112),6) = '201902') m1
ON m1.CaseNumber = a.CaseNumber --most current month
LEFT OUTER JOIN (
SELECT DISTINCT
[CaseNumber]
FROM
[dbo].['v2-2yrTotalDefault$']
WHERE
LEFT(CONVERT(varchar,[Month],112),6) = '201903') m2
ON m2.CaseNumber = a.CaseNumber --previous month
WHERE
a.[Month] > '12/01/2018'
GROUP BY
a.[CaseNumber]
ORDER BY
a.[CaseNumber]
/the continually errors out due to the following error:
Msg 8120, Level 16, State 1, Line 8
Column 'm2.CaseNumber' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.
Msg 8120, Level 16, State 1, Line 9
Column 'm1.CaseNumber' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause./
additionally with the above i don't want to have to hard code the months in the SELECT statement. I'd like to be able to control what month i'm looking to view in the WHERE clause.
in the end i'd like the results to return to columns, one reflecting previous months 'age' in months, and the second showing current month 'age'. if a CaseNumber dropped off prematurely, i'd like the current month to say 'premature_expiration'.