I should start by saying that I was unfamiliar with Sir Nicholas Winton until I saw the movie, so someone with greater knowledge about his life could nitpick on details. I found the movie compelling and heartwarming. In the movie, the young Nicholas visits Prague in 1938, and sees firsthand the refugee problem, especially the plight of children, and against a host of hurdles, manages to save hundreds. That is one storyline. The other is his later life, as played by Anthony Hopkins, where he is still dealing with the guilt/sadness over those he wasn't able to save, including over 200 children on a 9th train that was scheduled to leave Prague on the day the Nazis invaded Poland. Winton (in the movie) never sought notoriety, which only came through his efforts to find an appropriate place for the scrapbook with records of all the children he was able to save. The story is inspiring, and highlights what a handful of "ordinary people" can accomplish with determination and persistence. Could some of the characters have been more fully developed? Was the reunion thru the TV show a little contrived or melodramatic? Maybe yes to both, but whatever shortcomings the film may have didn't effect my enjoyment of the film.
In my opinion, this would be appropriate for middle school age and above since there's no offensive language or nudity. Imagine the conversations a movie like this may start (prejudice, discrimination, historic fact of the holocaust, the desire to help others less fortunate, etc.).