I have heard it said more than several times that the younger you start sitting cross-legged, the easier your body finds it to adjust. Well, I started when I was thirty five, after twenty or more years of avid running and cycling, activities which do not tend to promote leg bendiness. I managed to get one ankle above the other when I first started sitting, which I heard referred to as quarter-lotus, and like most people, found that one hip was more obliging that the other, so I tended to prefer that side. I was happy enough to go along with that at the outset of my practice.
In my first winter at Tassajara, in 2002-3, my body having adjusted to the amount of sitting that we do there, I found that I could get one leg up into half-lotus for a short time, and I was very excited. I kept working at it until I could sit whole periods like that, and tried to get the other leg to co-operate as well. This was going well until the year or two of emotional upheaval that I subsequently had, when everything in my lower back and hips seemed to tighten up and not want to let go; sitting became fairly constantly painful, even when I tried to add in some serious stretching beforehand.
It was only when I returned to Tassajara in 2006 that this started to reverse itself - there were a number of occasions when I would be sitting in the zendo and I would feel a jolt of energy emanating from my lower back, or sometimes from my knee or my foot; things were releasing in my body, and eventually I could do all my sitting in half-lotus with either leg - until the last sesshin I sat there in 2008, when my left leg completely locked up after I had twisted my pelvis, probably running up and down the Tony Trail....
Recently I tried once again to sit in full lotus, and despite it having previously seemed to be beyond my capacities, found that I could do it. Again, I tried a few times for ten minutes at a time, in the ino's office, and in the last couple of weeks, I have tried it out in the zendo during second period in the morning, which is just thirty minutes, and have managed okay. I think my pelvis is still a little skewiff, and the first thing I notice about being in full lotus is how good it feels at the base of the spine, much more spacious and relaxing than half-lotus, which makes focusing on breathing and the hara much simpler.
Now none of this is to say that you should sit in full lotus to have a proper experience of zazen; it's more like - if I can do it, and it has taken me the best part of eleven years, you probably can if you want to work up to it. The benefit of the position, as has been known since before Buddha's time, is that it gives you a stable base for the upper half of your body to be upright and relaxed, and that is the most important thing. It is just as easy to be upright and relaxed when sitting in seiza (kneeling position) or in a chair, or if you have good posture, in other cross-legged positions, like Burmese with one leg in front of the other. That last sesshin at Tassajara I mostly spent lying down, which made the periods go by so much faster, but I tried to stay focused in the same way. And as I heard someone say many years ago, sitting upright does not stop your mind from spinning off into thought or going to sleep, but it is a good place to start from and to return to.