We don't. River's appearances occur in a fairly random sequence in relation to her timeline. She was youngest in "A Good Man Goes to War", which took place shortly after her birth, & oldest in "Forest of the Dead", in which she died. In "The Impossible Astronaut"/"Day of the Moon", we saw her at at least 3 different ages: the young Melody Pond (1st incarnation), the adult River (3rd incarnation) in the spacesuit at the lake & the older adult River watching the shooting. In "Let's Kill Hitler", we saw her as a child & as a young adult (both of these being her 2nd incarnation) & as her freshly regenerated 3rd (River) incarnation.