The key to seiza is increasing the range of motion in your ankles. Definitely different people start at different places in terms of that range of motion, but unless you have an injury, you can eventually get there. A couple of quick tips: warm up before you do it -- even going for a quick walk will help. Don't force it. Try to stretch the ankles out a bit, but if it starts to get painful, stop. If you do it too much, you'll get inflammation in the joint, which will just make things take longer. Try to do seiza for short periods during the day. Ideally do it once an hour, but because you might feel self conscious doing it at work, etc, just try to do it whenever. Definitely every day. You can easily go from not being able to do it, to being able to sit seiza through a whole raft of boring speeches from the CEO at a Japanese staff party in only a few months :-)

Interestingly, the squat is a very similar problem, but in the opposite direction -- your ankles need to bend the other way. Lack of range of motion makes it so that you can't position your weight forward enough to stay on your feet. There is also the compounded problem of needing the flexibility in your hips.

Finally, sitting cross legged is also related to ankle range of motion -- but in a kind of diagonal way. To sit comfortably cross legged you have to be able to put the tops of your feet flat on the floor. Of course, this requires flexibility in your glutes as well, but if you don't have the range of motion in your ankles, your feet/knees are always going to hurt. A full lotus position is actually easier on your joints, but requires a lot more flexibility.

Anyway, like I said, there is definitely a range of starting places for flexibility for each person, but no matter where you start you can get there (barring injury, or other kind of unusual mechanical problem). Strangely, I find seiza to be the easiest of the three positions even though I initially found it to be the hardest (after you fix the range of motion problem in the ankles, there isn't really any flexibility you need).