Is the phrase 'aged appearance' idiomatic?
It could be, in the right context. In what context did you see/hear it?
Yes. Sometimes we have something new, but we want it to look much older - to give it an aged appearance.
There are techniques to do with with wood, metal or even paper objects. The process is called "antiquing."
Theatrical makeup also can give people an aged appearance. It is common for actors to play a character much older than they actually are.

Wooden chest with an aged appearance.
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