It’s more of an expression or a statement.. I don’t know the full context, but from the looks of it, an alternative translation would be, “she looks great for her age.”

"Look" is a "bidirectional word. It describes both an action of perceiving and also the quality being perceived. Here are some other bidirectional words: "smell" "feel" "taste" For example, a fabric can have a good "feel", and you can also "feel" it. A food can "taste" good, and you can also "taste" it. "Look" works in the same way. She can "look young" (appear to be young), and you can also "look at her". It is presented as a fact. She looks young right now. The grammar of your little sentence is deceptively complex. I just had a discussion with chatGPT about it, and convinced myself that there are many potential ways to analyze it. Being unable to choose just one way, I won't try.

Looking in this sense is appearance. She either is young, or did something/wears something to make her appear young.

This form 'is looking' is not how we would usually say this. Although the simple present, in this case 'looks' [young] is normally used to refer to something over a period of time ie she looks young, not just now but most of the time, in the case of 'sensory' verbs like 'look', 'feel', 'taste', etc [Jonathon's 'bidirectional' verbs] the simple present is normally used for what is happening now as well as more generally, For example 'she looks young in that outfit' could be either right now or whenever she wears it, and the context in which the comment is made should provide the distinction, so the answer to your question depends on the situation described.

"Looking" in "She is looking young" is a present continuous verb. - It means she appears young right now, not necessarily always. - It's a temporary or changing state, not a general fact. "She looks young" = A fact, always true. "She is looking young" = Right now, maybe due to makeup, lighting, or mood.