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I've done some work with the Ideation module in the past. I am not aware of anything in the indexing that would remove older content other than the reduced importance of those results due to age. Assuming the older ideas are still in a published state they should be indexed and available.
I would try rebuilding the index and making sure that you are running some test queries that are unique enough to guarantee a result. Always possible there is a but in newer versions or something has changed in the manager classes for the check on whether the item should be indexed.
This sounds like possibly a bug or environment issue. I would file a ticket with support to track down. Kevin is correct, as long as the ideas are still published then they should be in the index.