There's nothing wrong with sitting in a chair or kneeling on a bench or cushion, as long as your back is straight and unsupported.
It is worthwhile developing a cross legged posture if you can is then it will be a way of your body to tell your mind that now is the time for meditation, but you have to be realistic considering your age and physicality.
This video has a good series of stretches you can do http://www.youtube.com/watch?v=GS6ju33eeow
I did a similar routine every day for a month and haven't looked back since, but there is nothing wrong if you find you're not up to it and use other postures.
"Whenever we feel that we are definitely right, so much so that we refuse to open up to anything or anybody else, right there we are wrong. It becomes wrong view. When suffering arises, where does it arise from? The cause is wrong view, the fruit of that being suffering. If it was right view it wouldn't cause suffering." - Ajahn Chah
"Remember you dont meditate to get anything, but to get rid of things. We do it, not with desire, but with letting go. If you want anything, you wont find it." - Ajahn Chah