Who can do yoga?
Anyone, regardless of age, gender, and physical ability, can do yoga. Yoga is not a competitive exercise - the practitioner is not comparing him/her self with others OR with what (s)he "used to be able to do". In fact, although many Westerners come to yoga for the physical benefits, that is only one step in the ultimate goal of yoga, which is spiritual evolution. Mental evolution - learning to still the mind - is another step along the way. Each aspect of yoga has benefits, which is why it is suitable for all. It is, of course, strongly recommended that those suffering from a specific medical condition check with a doctor before taking up asana practice.
Benefits of Yoga
The asanas, or steady postures, benefit the practitioner on a physical, mental and "pranic" or life-energy level. Physically, asanas help increase and maintain flexibility of the spine, toning and rejuvenating the nervous system. The gentle movements bring flexibility to the other joints and muscles of the body, as well as massaging the glands and internal organs. Circulation is improved, ensuring a rich supply of nutrients and oxygen to all the body's cells.
Mental benefits arise because awareness and concentration in the postures free the mind from disturbances and promote steadiness of mind, which can help to improve one's outlook on life. The pranic benefits of yoga are achieved because the postures work in a similar way to acupuncture or shiatsu (stimulating the body's major pressure points), but are subtler. Thus, asanas must be practiced regularly over of time, and the benefits attained will be long lasting.
Spiritually, the yogic method provides guidelines for our evolution from self-awareness to awareness of the world around, and finally unity with the universe. Astanga yoga* means "eightfold path to unity", and these eight points are:
Yama (abstension) - truth, non-violence, control of sexual energy, non-stealing, non-covetousness.
Niyama (observance) - austerity, purity, contentment, study, surrender of the ego.
Asana - steady poses.
Pranayama - control of vital energy through the breath.
Pratyahara - withdrawal of the senses.
Dharana - concentration of the mind.
Dhyana - meditation.
Samadhi - the super-conscious state.
It's a lot to achieve, but it's a journey, not a race. Developing a bit of awareness of our thought patterns (which yoga helps us do) can go a long way towards finding true contentment, and to begin to reach beyond our daily concerns to a higher level of concern - for others, for our world, for the future.
*not to be confused with the popular use of the term "astanga yoga" which refers to the strong style of asana/vinyasa practice developed by P. Jois.
Preparing for yoga:
Yoga is best practiced on an empty stomach. Try to eat a light meal 2-3 hours before practice.
Dress in layers. Wear non-restrictive clothing. Have a mat or towel on which to practice.
Asanas are best practiced in bare feet; socks can be worn if necessary. Remove shoes at the door.
Maintain a peaceful frame of mind before practice, that is, when you enter the practice room, move quietly, speak softly, prepare mentally.