Geraldine I read it over my 13th birthday weekend and it had a deep influence on my subconscious. Reading it again in middle age I appreciated it differently.
Barbara Hard to answer; I personally would have loved it as a teenager, although I was in my early 80's when I read it.
Jeslyn As far as her early years and the VAD experience, I think that secondary/high-school (teenage) years would be fine to read this - it's the final portion, the political life and League of Nations stuff, that may be harder for readers that young to really sink their teeth into. I just didn't feel that the style of writing had a pace that advanced her thoughts well, and the habit of adding delicate description into sentences about political initiatives, speechmaking, etc. would not have been something I would have avidly read as a teenager - I struggled to stay focused as a grown adult!
Megan It kind of depends. While I wouldn't call the book overblown in its depiction of what she saw as a nurse, she is very frank. I would say that anyone with an adult reading level will be able to comprehend the words, but it would take someone in their late teens or early twenties to really be able to understand what she's talking about on an emotional level. I say this not in a patronizing way, but as someone who is in her twenties--I don't think I would have really *gotten* the book ten years ago, though my reading level would have let me understand it well enough.