Atrial fibrillation can occur in anyone at any age.  Babies are born with atrial fibrillation and some people live their entire lives in fibrillation.  It can at times be detected in fetuses in the womb (in which case the mother is treated with drug therapy passing the drugs across the placenta to treat the developing fetus).  Thus, atrial fibrillation can occur in anyone at any age.  Having said this, the majority of people who get fibrillation are older patients.  As the graphic above illustrates, the incidence of fibrillation increases dramatically with increasing age.  Although the total number of patients with fibrillation peaks in the mid 70's age range, this peaking is due to the fact that at more advanced ages there are fewer patients around to count.  If one looks at the percentage of the population at any given age who have atrial fibrillation, this increases with increasing age.  Thus about 7% of all 80 year olds have A. Fib. while over 10% of those over 90 have A. Fib.  Atrial fibrillation (as discussed in the section "what causes A. Fib.") results in most patients from stretching of the upper chambers of the heart.  As we all know, with increasing age, tissues of the body tend to become more lax and stretch.  For example, the tip of the nose slowly droops down the older one gets.  Other bodily structures also tend to droop with increasing age (we don't need to get into specifics here).  The connective structures of the heart likely stretch with increasing age and at the same time the prevalence of other factors contributing to chamber stretch such as leaking heart valves and hypertension increases with increasing age.  This thus likely accounts for the increase in A Fib. with the increase in chronological age. 

Risk factors for developing atrial fibrillation include advancing age, leaking or non-compliant heart valves and hypertension.  Some people will have severe frequent episodes of atrial fibrillation without a single identifiable risk factor.

In the majority of patients with A fib., the arrhythmia is recurrent and represents a lifelong problem irrespective of its age of onset.  In some patients however A. fib. occurs due to transient factors.  Atrial fibrillation for example is very common in the early period after open heart surgery affecting about 30% of all patients in the first two months post-surgery.  This type of fibrillation tends to resolve and not return.  Severe metabolic stress such as can occur in shock can also cause a transient self limited episode of A .fib.. Disorders of the thyroid can also rarely cause A . Fib as can inflammation of the heart from myocarditis (usually due to a viral infection).  Unfortunately, for most patients (>90%), atrial fibrillation is not caused by an easily reversible factor and once they have the arrhythmia, they tend to continue to have it for life unless something is done to correct the substrate for the arrhythmia.

Atrial fibrillation is either intermittent in nature ("paroxsysmal" fibrillation) or occurs continuous ("persistent" or "permanent" fibrillation).  In most but not all patients, atrial fibrillation starts as intermittent episodes of abnormal rhythm with periods of normal rhythm in between.  As the process causing this disease progresses, the problem becomes more chronic to the point that many patients with paroxsysmal or intermittent episodes of fibrillation will one day enter fibrillation and stay in the arrhythmia continuously thus becoming chronic or "persistent" fibrillators.  This is the natural history of atrial fibrillation; one of slow but inexorable progression with gradual increase in both the frequency and duration of episodes until it becomes a chronic continuous problem.

Atrial fibrillation is by far the most common of all the major cardiac arrhythmias with a frequency higher then all other potentially serious arrhythmias.  The graphic below illustrates the frequency of atrial fibrillation (denoted as "AF" in the graphic) versus the other serious cardiac arrhythmias seen by cardiologists.

One can see what a major problem atrial fibrillation represents; hundreds of thousands of patients suffer from this arrhythmia.  If your reading this web page and have atrial fibrillation, then you're in very good company with many, many other patients.