Why shouldn't an uncle be ten years only older than his nephew? Sir William's father might have married late in life or didn't have a son until late (remember, Mr and Mrs Bennet hoped for a son long). Surely, a 30-year-old man, mentioned that early in the novel, may prove a more useful character than a 60-year-old... :) Dickens uses the device of an uncle and a nephew being close in years with great success in "Edwin Drood", and points there, too, that in big enough families an uncle could be younger than his own nephew.