Determinants of self-rated health among an older Tibetan population in a Chinese plateau area: analysis based on the conceptual framework for determinants of health oleh Yajie Li, Qucuo Nima, Bin Yu, Xiong Xiao, Peibin Zeng, Deji Suolang, Ruifeng He, Zhuoga Ciren, Pingcuo Wangqing, Ciren Laba, Yangzong Silang, Ling Song, Yixi Kangzhu, Jingzhong Li