This is a question that is often asked (and hotly debated).  From a spiritual or philosophical perspective, the answer would seem to be "yes!"  It is for everyone in the sense that it is "the expression of love."  And it would also seem that everyone could benefit from attempting to refine themselves, using the principles of Aikido and Budo.

But  "everyone" is an all-encompassing word.  Not "everyone" has the mental faculties required to understand the meaning of a "principle" or of a "code."  Therefore they would be incapable of choosing to act based a principle or a specific code of conduct.  There are those for example, who are born with permanently diminished mental faculties who would probably not understand the meaning of a principle and would therefore not be capable of choosing to act based on a principle.

Furthermore, if we confine our discussion to the practice of the physical art (and Aikido is a physical art), then it again seems self-evident that Aikido is not for everyone. There are quadriplegics and there are those who are confined to a bed.  These individuals would not be able to practice the physical art of Aikido.  There are those with brain abnormalities that leave them with little control over their bodies such as individuals with cerebral palsy.  Clearly Aikido, as a martial art, is not for them.

But even if we set these exceptions aside for a moment, there is another sense in which Aikido is not for everyone.  Aikido is not for everyone just as Christianity, as a religion, is not for everyone; Buddhism is not for everyone; Judaism is not for everyone. And even formalized religion itself, is not for everyone.  If this were not the case, we would have one world-wide religion that all would practice.  (Akbar, one of India's Mughal emperors in fact attempted this on a small scale.  He came up with a synthesis of Buddhism, Islam, Hinduism, and Sikhism which he hoped would catch on as a religion that pleased all the major religions in India.  He did not succeed).

There is another sense in which Aikido is not for everyone.  It is not for everyone just as yoga is not for everyone.  Nor is Tai Chi. Some people simply don't take to the static postures of yoga - they prefer movement as a means to improved health and awareness (such as the Feldenkrais Method or Tai Chi).  Others simply don't take to the slow deliberate movements of Tai Chi and prefer something more vigorous like Aikido.  Yet others don't take to zazen or sitting meditation as a formal practice.  They prefer more active ways of concentrating the mind, increasing their self-awareness, or expanding their consciousness.  They prefer something with movement - the martial arts, for example.

The founder of Aikido himself seems to have suggested that Aikido is not for everyone (in the sense that I have spoken of it above).  Moreihei Ueshiba said: There are many paths leading to the top of Mount Fuji, but there is only one summit - love" (Stevens, p 42).  In other words, though the realization of the most fundamental universal principle - love - may well be where all of us are headed, we may choose different ways of getting there. 

One individual may prefer some form of formalized religion as his or her path to the "mountaintop."  Another may choose Yoga or Tai Chi.  Another may chose some form of formal meditation practice - and still another may chose Aikido.  Aikido, being a form of traditional martial art, realizes its paramount principle - universal love - through rigorous training of the body.  Not everyone has the capacity for -- inclination toward -- or desire for -- this physical rigor.