Simple, the Lex that is around now is a different body. Paunchy, bald Lex 1.0 died or radiation poisoning, and had his brain transplanted in Lex Luthor Jnr (A purpose built clone), when the clone body broke down and started to, basically, fall apart, he traded his soul to Neron for a fit, healthy body... only weird thing was he didn't ask for a full head of hair.