I'll try to take them in layers. The absolute pinnacle of benefits is simply the choice to pay more attention to one's own body posture. Much of our most challenging pains come from having ignored something for too long. Half the battle is not making stuff worse and that's cake when we develop the habit of feeling our posture. It is for this reason that 1 hour of Pilates can sometimes change a person's life. So, dodging or minimizing a phat list of spinal degeneration disorders is the 1st layer. As for the second layer of benefits, as a physical adult I derive a lot of real comfort knowing that some of my decline is reversible, not all of my aches and pains are inevitable, and in general, that some age-related problems can be fixed. Thoughts like that keep me warm at night. Yeah, but what about right now? Actually, the benefits of Pilates come really fast. The now fossilized old cliche "In ten sessions you'll feel the difference, in 20 sessions you'll see the difference, and in 30 sessions you'll have a whole new body!" is not so crazy. 10 sessions is ample time to teach good breathing skills that feed into postural improvement which then feed back into breath volume capacity. It's a tidy loop. More oxygen in the blood stream means some systems can work less hard and others benefit. Everything you love about yourself (strength, speed, memory, cleverness, even compassion) can be enhanced by more O's in the stream. Strong, consistent breath habits are raw horsepower for free.
20 (focused) sessions is enough to take it to the next level. While each individual Pilates exercise is therapeutic in its own way, there is a hard won synergy that comes from doing many of them in a row. By your 20th session you could have learned enough Pilates to spend a whold hour doing nothing but Pilates. Constant movement, constant engagement, constant breath. At this level ( not so easy ) people's bodies change fast.
The 30 session/whole new body thing ... well, a newly well aligned and tempered body, fueled by a newly increased VO2 max with (literally) new shock absorbers is close enough for me. I've gone pretty far around the block to say that Pilates can make you look and feel younger. Its true.

The bottom limit on age probably has more to do with comprehension than ability. Many of the exercises bring us into competition with some of our worst physical habits. For instance, hundreds tends to make a body want to hunch, jut, and arch different parts of their spine. A person is "old enough" when they can sustain an opinion about their own spine. In my experience, this kind of bodily awareness kicks in around the age of 11 but sometimes not til much MUCH later (oy). Weirdly enough, the upper age limit is much the same. As we age habit turns harder than healthy bone. Folks will report standing up perfectly straight from some pretty far off postures. A part from all the common sense aspects of aging that relate to all exercise ( consult an expert, don't guess) the big question remains can this person recognize and take responsibility for the exact position of their body. Older folks and injure of handicapped people can often exert safely and productively on the Pilates machines well with in their personal limitations.

It could be, if that video has layered goals. Pilates is as much about awareness as it is about exertion so, people need a chance to learn what to feel, and where before they exert themselves aerobically. Once that kind of exertion is taking place, we think and feel a lot less clearly. Since a video can never stop and fix something you've learned incorrectly, doing a good and thorough job of introducing the moves would be very important. Each move would need to be broken down and time spent on it before tying it into a whole flow. The video would almost need to contain a series of small classes leading up to a complete flow. I wonder where one might procure such a wholly complete video? Perhaps, it is coupled with a lively colorful manual.

Don't try to progress too fast. It's no exaggeration to say that I could fill an hour's lesson with 1 or 2 of the basic mat moves. Hundreds alone is a real bear of an essay. Its a self imposed progressively difficult pattern of simultaneous strain and release plus, its way harder than sit ups if you do it right. If the class seems too hard, you should feel a snobby superiority to those goons who think mere limb flapping is pilates. You are seeking something deeper. Take your time, don't worry about feeling behind or bewildered, it will pass.

The quick answer is 3 times a week. The long answer explains why. Things we do once a week or less are really only useful while we are doing them. Our goal is to carry the hard tissues(bone) using the soft tissues (everything else) in a new way. So, an infrequent session can help, but it won't stick as habit. We commit to use these arcane and archaic techniques to make progress. Even 2 sessions a week has the danger of constantly re-learning routines and moves. Getting through a routine isn't Pilates, getting through it retaining your form and breathing is.
3 times a week gives you a darn good chance to get past the cumbersome, necessary memorization stage so you can begin working on what you've learned. Does anybody think doing a Kung Fu routine once will make them any tougher? Does anyone think memorizing lines is acting? 3 times a week, y'all. Practice hard.

Maybe. If you can already assume the various ready-positions, then you can use Pilates to decompress your vertebrae some. Usually, this is a big improvement. Learn more. Don't push on parts that hurt unless you absolutely know why.

Sort of, but in 2 ways. Chances are, your look will be enhanced more by a good spinal alignment than by losing a handful of pounds. We are all "skinnier" when we are taller, its simple hydrodynamics. So, the basic skills of decompression bring huge benefits to how "skinny" we look.
At a more exciting, aggressive level, as you move into intermediate work it is far more challenging to keep pace with good form. Just doing stuff right and fast becomes an aerobic event. One of the few aerobic workouts that is actually good for your skeleton and its alignment. People who get to this level begin to change fast. Kicking your own butt to maintain form is Pilates at its finest.