Social Structures

Estes mentions the term "social structures" a number of times in the second chapter of Social Policy and Aging. Here's a specific quote:

A more recent dimension of this theory is the concept of structural lag, wherein social structures (e.g. policies of retirement at age 65) do not keep pace with changes in population dynamics and individual lives (such as increasing life expectancy).

Social Policy and Aging, p 30

Wikipedia has a good definition about it here:

Social structure is a term used in sociology and anthropology to refer to relationships or bonds between groups of individuals (e.g. societies). Whereas 'structure' refers to "the macro", "agency" refers to "the micro". (See also: Structure and agency)

In a general sense, the term can refer to:

• entities or groups in definite relation to each other,
• relatively enduring patterns of behaviour and relationship within a society, or
• social institutions and norms becoming embedded into social systems in such a way that they shape the behavior of actors within those social systems.

By the 1930s, the term was in general use in social science^[1], especially as a variable whose sub-components needed to be distinguished in relationship to other sociological variables.

Wikipedia

It looks like Estes is using the term in the third sense here - that of "social institutions and norms becoming embedded into social systems in such a way that they shape the behavior of actors within those social systems. I think she also means it in a bigger sense, that social structures can be institutional - such as companies having a standard retirement age of 65.