From what I've read, 18 to 20% Protein feed is fine for (egg layers) type of chicken, (male, female) chicks, pullets, and adults.
For a mixed ages flock including chicks. Nutrena Naturewise: Starter-Grower. Purina: Start and Grow, Purina Flock Raiser Crumbles. Are good options.
For a mixed flock...