I'm just guessing. But in ch 7.1 she was 4, if he was 10 he would be 6 years older than her. I don't think he met her again soon after he became Shade, so if she was 10 when they met, he would be 16 and thus, appear to be older to her. Again, I'm just guessing. But there are many holes waiting to be filled in the next chapters.
Maybe is like twilight.... where the child/monster grows until their 20's and then they stop growing. He just happen to be a couple years older than her, but eventually he will stay the same age// another possibility is that when a human transforms into them, their form is based on their mental stage. I cant wait for the writter to let us know~
I'm kinda confused with the timeline of this or their ages (?)
In the first chapter we see Shade already grown up when Tanpopo was a little girl but because of the flashaback in chapter 7.1 their age gap is not that big
Also for a moment I truly thought that Shade was inmortal but I guess fiends age just like humans