We don't just communicate with words. We also use gestures and facial expressions. If the person who said "this John looks like an age nine John" was pointing his finger at the photo as he spoke, then all is well. It is grammatical (the way I wrote it), and it makes sense. However, it would be an awkward sentence if the speaker were not emphatically drawing our attention to that photo.

Here are some more natural ways to express that idea: "In this photo, John looks like he's 9 years old (again)." "John looks like his 9-year-old self in this picture." "This photo of John makes him look like he’s 9 again." "John looks just like he did when he was 9 in this photo." "In this picture, John looks like he's back to being 9 years old." These alternatives all convey the same idea in a natural, conversational way.

Also, "He looks 9 years old in this photo" is grammatically correct, but it doesn't necessarily imply that he looks like he did when he was 9, but just like a 9 year old in general.

Hello. You could say it like this... - This looks like John when he was 9 years old. - He looks just like he did when he was 9 years old. - He looks like he's 9 years old again in this photo.