No. Glenn-san's initial interpretation is correct. Although I think ほど would be more close to ~のほうが than 若ければ若いほど, i.e., "the more~, the more" here. 若いひとほどマナーを知らない人は(年寄りには)いない is the one for Mike-san's.
Depending on the context, this structure cam mean "don't know much about manners comparing to the younger people".
e.g.
今度フランス料理を食べに行くのだが、若いひとほどマナーを知らないので、箸を使えるところに行きたいと思っている。
I'm going to go out to eat French cuisine, since I don't know about the manners as much as the younger people, I want to go to a restaurant where I can eat with chopsticks.