Anyone, everyone. Age, sex, health, religion, physical or emotional handicaps are not barriers, nor is doubt.
The youngest I have attuned (at the request of her mother) was 6 months old. The youngest I have attuned who asked me was three years old
(his mother and father agreed). The oldest I have attuned was 96.
Reiki is not a religion; it has no strange dogma,
no religious tenets; it requires no rigorous ascetic practices or rituals. People from all religions, as well as atheists and agnostics use Reiki to help themselves and others. Most report the practice of Reiki deepens whatever religious beliefs they already hold.
I have attuned people unable to use their hands, amputees without hands, arms, or legs, paraplegics, those permanently bedridden, and others with severe handicaps. Channeling the energy sometimes requires a "work around" method, but we have them and all can do it. I arrange instruction and attunements to suit the time, place, and situation.