Pilates is not as easy as it looks if only because your brain has not been keyed into the new muscle groups you will be using. This makes it both interesting and challenging; one of my students calls it "the body exercise that works your brain." Because you have to keep so much in mind at once - breathing, movement coordination, correct head position, loose shoulders, contracted abs, turned out feet, etc etc - you really do need someone to coach you in the initial stages. You can learn the moves from books and videos but they can't watch you executing the movements and correct you.
In my own teaching I emphasize breathing... sounds simplistic, but the best way to access those lower abdominals is through an intense exhale (my students vocalize this) which forces the muscles to contract. Coordination is key: timing the length of your exhalation to coincide with the movement of your limbs. Gradual strengthening is also key: students performing beyond their flexibility limits only results in frustration or abandoning the program altogether.
Since many people cannot practice Pilates and get to the gym, I've added optional weight training into my routines. Using 3, 5, or 8 pound weights (women) and 8, 10, or 12 pound weights (men), students execute arm exercises with added resistance to strengthen and tone the biceps, triceps and even the delicate wrist and forearm muscles (excellent for staving off carpal tunnel). Moreover, by employing Pilates breathing and positioning while using weights, students learn to use the upper back and arms rather than the neck and shoulders.
This in turn leads to fewer stiff necks and headaches.
Almost anyone of any age can begin Pilates. My oldest client is 78 and has noticed (so have we all!) a marked improvement in her strength and coordination as well as a straighter spine.