Although I'm young I feel the pain of seiza too particularly because of an old football (soccer to you confused americans
) injury on my right ankle.
Don't be obstute and force yourself to sit seiza when its painful. That can lead to permanent injury. As with all stretches, it takes time. For me 2-3 years has brought about some improvement but no where near the comfortable zone yet.
A good stretching exercise to do would be:
1. Stand up straight, bow down and hold your knees. Then slowly flex the knees in very minute circular fashion.
2. From the bent position above, kneel down with your heels slightly elevated. Continue for sometime.
These two exercises would probably bring some flexibility to your ankles before you try sitting Seiza.
Another thing, massaging with hot oils might be a good idea.