I personally ship Magolor and Kirby as people around the same age. Namely in their late teens or early twenties. I know that a decent of people are wary about mahokabi, but it's not the case that it's automatically a ship between a grown adult and a small child.
The thing is that Kirby has not been given an official age in the Japanese games. They have only been described as a young traveller in the Japanese manual of Kirby's Dream Land.
The term for young person being used,若者 (wakamono), is vague enough that it can both be about teenagers and young adults. For example in this study about Wakamono Kotoba (young people language) mainly used web blogs "that were written by young people from their teens to their twenties".
Because of that, Kirby's age is treated as more ambiguous in the Japanese Kirby fandom. If the people that used the hashtag #うちの子のたや紹介 on Twitter are a good representative, then the most common age range for Kirby headcanons is that they're in their teens, though both young adult Kirby and small child Kirby are also common.
In Magolor's case, I have heard before that Magolor's speech patterns sound more childish in the Japanese script, compared to the English one, but I can't comfirm that.
As for why I personally lean towards them being close in age? When I first got into the games years ago I wasn't aware of the fandom yet, but I did watch some Kirby Café fan animations (by VTAnimation) and read a doujinshi called I'm Sorry About Lying (by 砂糖). Both in which in I got the impression that Kirby was supposed to be adult. These ended up influencing how I saw Kirby, especially the latter one.
I do also think that it would be cool if Kirby and Magolor were foils of each other. They are both travellers who aren't native to Popstar, but through the different circumstances ended up very different from each other. But I'll get more into that when I'm going to write my ramble on the ship essay section.
Once again, I get why it's so disliked in the western fandom and you do have to be careful when searching for it, but I did want to show that it's not universally the case, before there are any more misconceptions.