There are several variations of reiki and energy work, but most are based on the ability of the practitioner to touch the universal energy that helps balance the energy and energy centers (chakras) of themselves or others. A person receiving reiki or energy work is not taking or being given the energy of the practitioner. The energy is universal energy and the practitioner is the conduit that connects the person with this universal energy. All you have to do is lie or sit (depending on location) and relax allowing the energy to do its work.
While reiki and energy work does not necessarily feel individualized, it is. Prior to a session, you will sit and discuss your session goals with the reiki practitioner. During the session, the practitioner will be intent on your goals, but know, that sometimes the energy goes where it is needed rather than where the person requests. The body and the universal energy know where balance is really needed and that's why the goals do not always mesh with what happens.
Just like aromatherapy, reiki is for any one - including animals - and all age groups.