Aging
Strictly speaking, the effects of Aging, that is, the process of getting older, affects mortal beings as soon as they are born. However, in RAW, the negative side effects of Aging do not become noticeable before the age of 35. This is when the bodies and minds of mortals slowly but steadily start deteriorating, a process that is expressed through Aging Points, and, eventually, via Decrepitude.
Aging and Magi
Magi in Ars Magica can expect to live much longer than ordinary humans due to their magical Longevity Ritual. The extended lifespan comes at a cost, though: a magus's Longevity Ritual causes Warping and, eventually, Twilight.
Analysis of Fifth Edition Aging Rules
A thorough, though unofficial analysis of the Fifth Edition aging rules by Andrew P. Smith is published at Durenmar.