This book is very fast-paced. That's probably because it's a novelization of a movie - a movie simply doesn't have as much time to develop backstory or detailed characterizations. A book can easily devote four scenes to what a movie has to get out in one. Of course, a novelization can easily expand on a movie (the reverse of the distilling that normally happens when a movie's made into a book), but Asimov probably felt he couldn't change anything significant without talking to the movie producers first.
So, don't open this book expecting any great depth. The premise touches on numerous major issues: during the Cold War, Dr. Benes, a scientist who's helped develop the new science of miniaturization - shrinking something to microscopic scales without any loss of detail - defects from the Soviet Union to the United States. A Soviet agent tries to assassinate him; Benes survives, but a blood clot in his brain will kill him soon if not removed. So, our protagonists must be miniaturized and navigate a submarine through Benes' bloodstream to destroy the clot. All of this is moved over quite quickly, though, and the rest of the book follows our protagonists inside Benes' body. This probably came from the movie. It reminded me most of a movie I'd seen: Pearl Harbor is merely mentioned in the opening crawl; the film itself talks about the reaction in California.
That movie told a fun story, though, and this book does it even better. The moment I saw the protagonists almost get seasick from random Brownian motion of air molecules buffeting the submarine around, I knew Asimov would use science to tell a good story. And he does: each hazard they face in Benes' bloodstream is plausible and exciting. You needn't be an anatomy major to read this; as long as you know that the heart pumps blood around the body, Asimov will explain enough for you to enjoy this fast-paced, exciting book.
It isn't just a travel story, though, even through such an unorthodox clime. Our protagonist has reason to believe that one of the people aboard the submarine is a Soviet secret agent who'll try to sabotage the mission. But whom? I kept watching and waiting for the saboteur to be caught in the act. Eventually, he was. But the protagonist was ready for him - why, he is asked? Because I already knew who he was, he responds - and then proceeds to explain how he kept watching through the voyage, picking up on clues which seem so obvious in retrospect but I totally missed at the time.
Anyone from age ~12 up, I'm convinced, can enjoy this fun tale. No warnings apply, unless you feel the saboteur's death (after he's already tried to kill the protagonist) deserves one. Don't expect it to be deeper than it is, but have fun with what Asimov did write.