The correct answer is Option D - Population.

Age structure refers to the distribution of individuals of different ages within a population. It can be represented geometrically using age pyramids (also known as population pyramids), which are graphical illustrations that show the distribution of various age groups in a population, typically with males on one side and females on the other. This distribution can give insights into the reproductive status, growth potential, and social dynamics of the population.

A biotic community (Option A) consists of all the living organisms in an area, often including a diverse range of species, where the age structure of each species would be considered separately rather than in aggregate.

Landscape (Option B) refers to a larger area that includes one or more ecosystems, encompassing both biotic and abiotic components. While age structure is relevant to the populations within the landscape, it isn't a characteristic that describes the landscape itself.

An ecosystem (Option C) is a community of living organisms in conjunction with the nonliving components of their environment (like air, water, and mineral soil), interacting as a system. While populations within the ecosystem have age structures, the ecosystem as a whole is characterized by energy flows and material cycles rather than the age structure of its constituent populations.

Population (Option D), however, is a group of individuals of the same species that live in the same area and interbreed, producing fertile offspring. The term "population" directly refers to the individuals to which age structure can be applied, making it the characteristic feature of a population among the options given.