At age 60, when many of her friends are considering retirement, Josefina Luna is chair of the board of CERO Cooperative Inc. CERO is a five-member worker-owned cooperative on a mission to encourage composting and create jobs in the hard-up Boston neighborhoods of Roxbury, Dorchester, and East Boston. It's a small but unusually diverse team: Of the five worker-owners, two are African-American, two are Latinas, and one is white. They communicate in English and Spanish.