Okay, so I think I have the query down (not just close like last time I did a Data Explorer query)
Select 47 - Datediff(month, p.CreationDate, Current_Timestamp) as Age,
convert(DECIMAL(10,2), sum(case when p.AcceptedAnswerId is null then 0
) / count(p.Id) as PercentAccepted
From Posts p
Where p.PostTypeId = 1
Group by Datediff(month, p.CreationDate, Current_Timestamp)
Order by Datediff(month, p.CreationDate, Current_Timestamp) desc
should get it for you by month. The
47 - business was just adjusting the data so that the graphing wouldn't be in reverse (since older posts would have larger ages). Query can also be found here.
I'm assuming you wanted a graph, so if you go to the link and run the query, there'll be a "Graph" tab that'll let you see the results plotted. The graph is also reproduced below for convenience:
Judging from the graph, you're right. The percentage does seem to be dropping; it even dropped below 50%.