Age (Model theory)

In model theory, a branch of mathematical logic, the age of a structure (or model) A is the class of all (up to isomorphism) finitely generated structures, which are embeddable in A. This concept is central in the so called Fraïssé construction. The main point of this construction is to show how one can approximate a structure by its finitely generated substructures. Thus for example the age of the dense linear ordering without enpoints (DLO), is precisely the set of all finite linear orderings, which are distiguished up to isomorphism only by their size. Thus the age of DLO is countable. This shows in a way that DLO is a kind of a limit of finite linear orderings.
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