An English Descriptive Word for Two Words

I'm currently after a synonym for "middle-aged". However, I'm needing the word to be appropriate to describe a non-living thing.

Middle-aged is more or less associated with animals and humans, not particularly describing research or articles.

If anyone has any suggestions other than "midlife", please comment.

Thank you, everyone, for your help.

The stage of forest growth between a young forest and an old-growth forest is called a mature one (look for details here)

I believe that the same word can be applied to research. Here is an example:

In many cases, this means that only mature research topics are considered appropriate for serious research inquiry. When research topics have matured, there is usually plenty of data...

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