I taught GREAT EXPECTATIONS to my advanced eighth graders for many years. It's my favorite Dickens book, and the film version, while dropping a fair number of subplots, is most masterful in retaining everything that's important.
I had no trouble with John Mills as the older Pip, but Valerie Hobson seemed WAY too old as Estella. Martitia Hunt was perfection as Miss Havisham. In fact, apart from Hobson, I think the movie is perfectly cast.
BTW, Jean Simmons, who makes a grand young Estella here, played Miss Havisham in a BBC miniseries of the novel many years later.