I think that there are two things that have to be taken in consideration:
a) age plays a big role in Korean culture (hierarchy, respect etc.)
b) maturity
sure maturity is something that is highly personal, I´m not judging Momo here or even anyone in particular, BUT when you picture an AVERAGE (please, don´t use the whole "but I´m not like that") 24-year-old and an average 38-year-old ... they are both somewhere else.
A 24-year-old is someone who might be freshly out of college, still unsure what she wants in life, starting a new job, finding a new flat, not knowing how to fix a broken sink.
A 38-year-old man might have had several serious (that´s important to note, several SERIOUS relationships) and might want a family (might even be in a bit of a hurry to have kids (and men have their biological clocks too) and is settled and is sure in his work.
Now, to be concrete I don´t know Momo, so it is hard to judge. She is obviously rich, independent and successful.
But despite his comic acts, Heechul seems like someone who is actually pretty settled down, intelligent and knows what he wants. He is quite serious when it comes to work, so I imagine he is not much of a fool when it comes to relationships either.
If they fit, they fit! The best to them and I hope they can make things work!
But I was simply trying to explain why some people might be sceptical. (hate would be a bit of a strong word)
Also the fact that idols are not supposed to date might be another thing.