Putting “old” “young” and “adult” together in a row sounds very strange. Hyphens glue words together and help remove any potential for ambiguity. Here “six-week-old” and “young-adult” would mean that your readers would understand the sentence at first reading.

This page goes into more detail.

A little while ago, I made a quiz on common sci phrases – check it out here.

If you prefer, last week’s toon is here It involves mice and is more amusing.