The trouble with sitting to garden is that you're 'side-on' to the work and cannot use your body fully.

The problem with 12" high is that it doesn't come at a useful bendy bit of your body.

Other than that - boulders do sink over time as you raise the soil level. They make lovely habitats for nibbling insects. Perennial weeds wiggle underneath and snigger as you try to root them out.

If you were to think about a 'double wall' of rocks - even like a dry stone wall with a gap between for a soil infill, set so the inside space is about 3' across and set to catch the sun. About as high as where your legs join your body. That way you can lean against it while working.

Table top gardening using something like glasshouse staging for growing the smaller versions of vegies. I've noticed that many of my senior gardener friends are growing more of the gourmet foods than the bulk 'feed the family' crops. Many of them don't need the sheer space required for crops such as potatoes. Cut and come again crops don't need vast space - just good soil, warmth and reliable water.

Aside: if you think about installing a covered area and hard paths now you'll be willing to go out to garden for much longer, even when less agile than now. A glasshouse, or protected area, is wonderful for pottering and thinking and tinkering. Just be sure it's big enough to allow for a walking frame or seat. It probably won't need heating if you have it set to catch much of the winter sun.

As soon as you're comfortable with the notion - look for ways to do your vegie gardening in 'unconventional' ways to get the crops you use the most.

And, if you're planning to have fruit trees - think dwarf and espallier - and making them part of your landscaping layout as attractive, airy space dividers. Much easier to care for and harvest from.

I live in the Ozarks and all my beds have boulders and larger rocks for sides of my beds. I do wish i had made beds narower(Maybe 2') and maybe abt 2' high. All my paths are wide enough for a wheelchair too. Rocks do retain heat and help in cold weather but don't seem to hurt in hot weather. I have arthritis in hips not spine.
vickie

Most of the information I have read about raised beds for greater accessibility say 24" (for sitting) to 30" high (for standing) and 2' across if you can only access them from one side or 4' if you can access from both. Vetivert8 made a good point, if you aren't careful and make it so you are sitting sideways, it might hurt your back more after a while. If your hips, knees, ankles and feet are fine, you might want to consider building the raised bed high enough for you to stand and access it comfortably. Or consider whether sitting on a seat in front of it might work well too.

I suppose it would make more sense to start shorter and if that doesn't work out, you can always add to the wall.

oaklief is right, you shouldn't worry about the boulders burning up your plants in the summer.

You might want to lok into the "square foot gardening" forum. This is good for not having to reach alot.
I built my raised beds before my back went whacky and had to put a path down the middle of the bed so I wouldn't have to reach as much.
Also think about taller plants. My tomatoes were way above my head in the raised bed, resulting in reaching, which is a no no! Sigh.
So, my suggestion is to figure out what veges you want, figure out how much you can reach in, figure out how much you can reach UP, and then HIRE someone to make it happen! LOL