I have noticed both in myself and others, that as we age certain things either worry us more than they did in our earlier years, or suddenly start to worry us.
You have always been afraid of heights and now it is getting worse.
I am not qualified to say whether this constitutes mental illness or not, but I very much doubt that it does. At the back of our minds is the thought that a fall now (over 70 for me) is likely to have far more serious consequences than it would have had fifty years ago. Another thought in the back of our minds is that we want and hope we will be able to look after ourselves for the rest of our lives, but there is so much focus now on age-related difficulties and illnesses that we are bombarded by terrors every way we turn.
May I ask, have you had your eyes tested recently? If not, it might be a good idea, because a deterioration in ones sight can lead to a failure to estimate depth correctly, thus making us unsure, as to what we actually see. It may be something as simple, these days, as a cataract causing wonky depth perception.
If you frequently have to walk, drive or cycle along a narrow country road or lane with a drop (fenced or unfenced on one or both sides) your fear is not really irrational. If you are having difficulty assessing whether a kerb is 2 inches or 6 the fear is not irrational either.
Most of us feel chary about climbing ladders, taking down curtains while balanced on a step-ladder, alone at home now - we didn't when we were thirty, but now commonsense tells us to take care or delegate the task to someone younger.
If you are seriously worried about this: consult your doctor - the sooner the better. There is no point in worrying about something that may be nothing, but you will continue to do so, until you know it is not a serious problem.