Many societies are facing a big demographic shift, reflected in a fast growing generational unbalance with very high percentage of old people, which puts their social security systems in danger. In this context, a conceptual architecture for an ambient assisted living ecosystem is introduced with the objective of facilitating the development and provision of technology-supported integrated care and assistance services for senior citizens. These services are the result of collaboration among various stakeholders, enabled by the notion of collaborative ambient assisted living ecosystem. A 3-layered model is adopted for the architecture: Infrastructure layer, Care and Assistance Services layer, and Ambient Assisted Living ecosystem layer. The involved actors and their relationships are modeled as part of the architecture implementation specification. An example of service design in this context is also presented.