Squash is an intense sport that demands agility, coordination, precision, and good aerobic capacity. Because it involves multiple short bursts of activity, and upper arm exercise, it does elevate blood pressure during the periods of intense exertion. For people who already have high blood pressure, or those who have or are at risk for heart or brain vascular problems (IE, chest pain, heart attack, and stroke), squash could be dangerous.
On the other hand, it is probably no more dangerous than other kinds of upper body exertion -- shoveling snow comes to mind -- that can trigger a preexisting problem.
So, I would not let age over 40 stop you from playing squash unless you are at risk. I would recommend that if you don't know your blood pressure, haven't had your cholesterol checked, or have a family history of heart attacks, strokes, or diabetes, that you get a check up before starting any intense physical activity.
And of course, if you aren't already doing it, find ways to move to an even more healthful (and low fat) diet, reduce or manage the stress in your life, and quit smoking!
As an avid squash player who is over 40, I can attest to there not being a problem. However, I also regulary exercise (minimum of 30 minutes every day - either running, tennis, squash, or hockey) and generally watch my diet (although am not anal about it). I also do not fall into an at risk health category.
Bottom line, check with your doctor first but age should not in of itself be a limiting factor to playing squash.
p.s. squash though is hard on your body, not just your heart....but it is a great sport!
I am 49 and have played squash since I was 18 - I get great enjoymeent from playing and although not as fit as I was I still maintain its a great was to keep in shape. I have never had any problems other than a few torn muscles though not warning up properly. I also play admington and find that equally as challenging. My doctor says that as I have been playing for so long its not really a problem however great care should be taken if your thinking of taking it up post 40 thats when in can be dangerous.