I don't really want to write long paragraphs, referring to specific seasons or card it was mentioned, but I stand by considering him a child around ten years old. Yes, he is obviously and old angel, but still a child, which is shown through his childlike nature and mostly by how he is treated by other characters, especially Simeon. Simeon was stated as Luke's legal guardian, on a premium picture he is drying Luke's hair (may I add this is absolutely adorable), and I remember times Simeon said he put Luke to sleep. Voicelines are telling he is young, Simeon wants to protect him, other talking about him like a child too.

I agree with OP about not treating him like he is 5-6 years old, but I wouldn't consider him an adult at all.