The science-fiction novels of Robert A. Heinlein cover a wide range of subjects, but most of them have something to do with the future - that is to say, some possible future of the human race. And although some of them are appropriate for younger readers, many of them are quite adult. Do observe the "recommended age" notices in the following notes on certain Heinlein selections that I have read.
It also bears noting that Heinlein's work - sometimes subtly, often not - tends to subvert traditional, Judeo-Christian morality in a manner that, in my opinion, smacks of Fascist ideology. For example, while Starship Troopers is one of his books that I have not read, my immediate to the film based on it was: "This is the most Fascist movie I have ever seen." (In fairness to Heinlein, however, I have the same quibble with several other films by director Paul Verhoefen). You may come to a different conclusion, but I mention this up front so that you needn't be surprised.
Red Planet (12+) - In an age when humans are colonizing Mars, a good-natured boy named Jim Marlowe forms a fateful friendship with a furry basketball named Willis - a spherical form of Martian wildlife that can put out various appendages, including eyes on stalks, as needed; and that can parrot back things that it has heard. Together with Jim's friend Frank, the pair gets lost in the harsh Martian landscape, encounter an alien culture, stand up to human villains, and save the future of mankind.
Have Spaceship, Will Travel (12+) - Stand by. I have this book somewhere on my "getting around to it" shelf. When I have read it, I'll try to say something intelligent about it right here.
Stranger in a Strange Land (17+) - This book was one of the cult hits of the 1960s, appealing especially to the Baby Boomers who, especially in that decade, delighted in challenging the institutions and values of our society. In the book that put the "grok" in the slogan "I grok Spock," Heinlein depicts a man raised on Mars, by Martians, who returns to earth and challenges the human race to change. Valentine Michael Smith has learned a different set of values - as well as some amazing abilities that perhaps lie dormant in the human race, waiting for us to learn to use them. In order to transform mankind, Smith starts a new religion (skewering, in the process, another fictional religion that bears at least a passing resemblance to one or two real ones), gathers a following, and in the end makes the ultimate sacrifice to ensure that his plan to save the world will go on. Along the way, brace yourself for some mature content as Heinlein tweaks our culture's sexual and dietary taboos.
Friday (16+) - Friday is the name of a girl who, in a grim vision of earth's future, has superhuman abilities and yet lives the life of a second-class citizen because she was genetically engineered. She makes a living as a top-secret courier, smuggling objects and information by way of a container surgically implanted in her abdomen, running errands for her top-secret "Boss," and trying to keep up appearances as a normal human being during her off-time. Be prepared for disturbing scenes of violence (including, I'm afraid, a gang rape) and other things that tickle our society's mores, sexual and otherwise; but all within a cleverly-plotted, action-packed adventure in an intriguing futuristic world.
Farnham's Freehold (16+) - Old man Farnham may have been thought paranoid for building a well-stocked bomb shelter in his back yard, but one day the unthinkable happens and a missile scores a direct hit on it while Farnham, his family, his daughter's girlfriend, and a chance visitor (who happens to be black) are inside. The unexpected result is that the bunker and those inside it are blasted thousands of years into the future. They come out into a world ruled by a black Muslim culture who treat white people as cattle, to be used as slave labor or even as food. Once again, Heinlein's story serves as a bully pulpit for his controversial views on sexuality, race, religion, the value of life, and other values. But he also tells a remarkable tale from which, years later, I still enjoy an occasional chuckle at the coded letter Farnham sends to his mistress (From memory: "Luba, ya bin smokin komplott seit Hector was weaned...").
Other works by Heinlein (which I haven't read) include The Moon is a Harsh Mistress, Time Enough for Love, and The Cat Who Walks Through Walls, plus numerous short stories, including one personal favorite, "All You Zombies—," which poses the (to me, at least) unanswerable question: "If a transsexual time traveler turns out to be his own father and mother, then who were his grandparents?" Other than this one story and the previously summarized novels, however, I haven't read any of Heinlein's other works; so my mentioning them should not be construed as a recommendation. The scuttlebutt on the books I haven't read, which jives with my experience of the ones I have, is that Heinlein tended to start strong, from a narrative viewpoint, and lose focus later on, so that each book progresses from a tight storyline to more of a philosophical treatise. Even disagreeing with his philosophy, however, you may enjoy his work. So start with his books that I can recommend, and if you like them, you're on your own from there.