Physical balance does get more difficult to do as we get older. Balancing postures require a lot of practice, and require us to be patient with ourselves. Many of us try to jump right into a balancing pose we've never done and then give up when we realize how difficult it is. The key is to start really slowly, and maybe use the assistance of a wall or a chair (depending on the pose). If you can find balance in a posture, you'll find that it brings you right into the present (which is what Iyengar is saying in his quote). There is no way to really balance if your mind is all over the place. 

If you want to learn tree pose for instance, try doing it for just a couple minutes each day for a couple weeks and see if you can start to get it. It doesn't take a lot of energy and it's fun when you do start to find your balance. First you have to find a point of focus--"drishti" in yoga. Look at something in your room or environment that is not moving. A point on the floor a few feet in front of you or whatever. Start balancing by just bringing the heel of the foot off the ground on the leg you're bringing up. Then slowly see if you can progress to putting your foot on your calf. You can also start to learn your way into tree pose by having your back up against a wall before you bring one heel (or foot) up. 

BTW... I'm working on ideas for a whole lesson/class on balance. 😉

"Balance in the body is the foundation for balance in life. In whatever position one is in, or in whatever condition in life one is placed, one must find balance. Balance is the state of the present - the here and now. If you balance in the present, you are living in Eternity" - B.K.S. Iyengar, Light on Life