We can start CMMI Implementation depending on the organizational need. Once an organization starts growing in size, it starts feeling the need for effective processes and the use of processes by the people. This is the point of introspection - and the organization starts looking for some model. The question is Which Model. CMMI is one of the best suited and well researched model to be used for institutionalizing the best practices for software engineering body of knowledge - which means requirements management, project management, design, coding, testing, delivery and maintenance etc.
You can start CMMI Implementation from today. However, it would be a good idea to plan the roadmap for the CMMI Journey from launch to successful implementation to successful assessment to successful sustenance of the system post assessment.
It also good to find out what are our organizational goals that we would like to achieve while implementing CMMI Process Areas.
My sugggestion would be to develop a detailed roadmap, explain what is expected from each stakeholder - management, coordinators, practitioners, project managers, project leaders, programmers, etc - and then begin the process. Good luck.