>>27735220Eevee is the adolescent form. There ia no adult Eevee: it just evolves, either while still young, with a stone, or when mature, through finding happiness. Pokemon lifespans are of course not 1:1 analogous to humans, but presumably if an Eevee remains unhappy, underused, or unexposed to harsh environments, it remains stagnant, unevolved and juvenile well into its later years.
