Merry and Pippin are upper class though, and upper class boys never grow up until....well, they just never grow up! You only have to look at Boris Johnson. I'm sure so many people voted for him just because he's amusing and a buffoon
Otho and Lobelia are nouveaux riche though, a bit like the Posh and Becks of The Shire. The sort who would think nothing of decking a child out in Gucci and diamond earrings so he/she looks "all grown up". So they might have been raised from a young age to be materialistic.
Bilbo and Frodo on the other hand are traditional middle class and all that kind of thing is "stuff and nonsense". Harumph. *shakes Daily Mail in disgust*