I have heard this from a yoga teacher before- it goes something like this:
Everyone who comes to a yoga class is different, some people are going to be great at poses and some other people are not going ot be quite as good. You shouldn't spend the class comparing your asana to other people's. You must focus only on yourself and that is the best way to prevent injury doing yoga.
I am in my 20's and do you think I can do downward dog with my heels touching the floor? I don't think so. Are there girls in my beginner's yoga class who do it with ease? You bet. But, I don't let this bother me.
Keep up the good work, yoga is very rewarding.