As a response to the confusion* I had when my players were creating characters, here is a summary how XP is calculated for characters in Ars Magica; using a slightly different method from the default in the style of explanation. The outcome is the same, and I find the points breakdown far easier to spend during creation, especially when done by hand.
As there are two broad character types in Ars Magica, the character generation is also designed in two ways: there are rules for Magi, and rules for everyone else. The Companion and Grog (mundanes) rules are very simple, and the Magi rules are far more complicated.
XP in 5th edition has been heavily base-lined, so that the points spent in Abilities (skills) are the same as the points spent on Arts and Spells (magical powers). The characters spend them in different ways, but essentially as the points are the same, I sum up the totals, and set checkpoints for the minimum spend in the different areas.
The examples below for Magi demonstrate how powerful having post-apprenticeship years are before play begins, so it is worth noting that it is not typical to do this for young Magi, or that it might create an imbalance between characters. Be wary allowing too many years post-apprenticeship, and always enforce the Warp point penalties (2 per year post-apprenticeship).
XP Formula: Early Childhood + Later Life (+ Apprenticeship + Post-Apprenticeship)