Characters in P&P part 1

My initial response to watching the first half of the mini-series would have to be how the characters would be portrayed. I had never seen any type of adaptation of the book, and have read it enough times to really have solid portraits of all of the characters. To me, the characters in BBC's adaptation were much older and often more over-the-top than I had interpreted them to be in the novel. Personally, I felt that, while the actors portrayed the characters well, their age difference than in the novel threw me off, and changed some context for me. First of all, I never imagined Jane and Elizabeth to look or act so much older than I was expecting. Second, although Mr. Collins is a "silly" man, I never expected him to be so awkwardly comedic in this adaptation. Although this physically does not change the story, personally, it changed the way I felt about certain characters, whether I thought their actions were justified or not.

Questions:
1). Did the change in age, (because come on, they do not look 19 and 21 years old) effect how you viewed decisions and actions?
2). Is there a reason why the BBC may have chosen to cast actors that would be older? i.e. How would the Lydia-Wickham affair be altered if Lydia really looked as young as she is in the novel?