I dont really agree with this because if firstly if implied, wont it mean that Khun who was in the early parts of the webtoon should seem older than his other peers during the tests since he had more mental development between them?

Since you say mental development affects your physical age appearance and actions this should imply that if you have more mental development you should age appearance should increase more. If that's the case wouldn't Bam have grown far more than Endorsi for example as well as his other companions since he had such a greater mental development change? But they grew at the same speed and similar age for some reason. Also how about Hwaryun? She seems even more mentally developed than Bam and Khun with how well she does her job as a guide and is coolheaded and even wise and yet she is of similar age appearance of the two and her other peers. And the whole group all grew at similar speeds and age appearances desipite having varying mental developments.

I feel like SIU makes people be the age he wants them to be and if not he probably would have clarified this aging system at play in the story earlier.