My 'why is Beelzebub's design an older man' thoughts have Occam's Razor explanations of:
Helps to visually distinguish him from the rest of the Sins.
The title 'Lord of the Flies' inherently brings to mind at least an adult figure.
If he was as baby-faced as Satan and Belphegor his depowered form would barely look any different.
And more actualized explanations of:
An older, larger body is easier to eat large amounts of food with. He can be a baby and chomp on people but that stomach is small and he'll fill fast.
The look of an older gentleman fits his want to seem like a connoisseur. This one is I think the big one, since Beelzebub wants to look like he's experienced.
It also fascinates me how much Verge's magic is candy-associated, since as a Witch of Gluttony-- candy's the epitome of gluttony. It's a sweet treat that doesn't have nutritional value, it's eating for the sake of eating, and the childishness motif that follows through from Beelzebub manipulating Leah follows through on there. Kids love gluttony, really. They eat junk food and want junk food, and isn't junk food just a great example of stuffing yourself without purpose? Maybe that's why Beelzebub goes near the young.











