Cry havoc! And let slip the pigs of war!
In ye old days, the assumption was a wizard was in their late 30s to start! (Old AD&D age tables).@Chaosmancer Because I'm apparently a glutton for punishment...
How many years does it take to become a practicing Wizard? To go from "I literally only know how to read and write" to "I'm a 5th level Wizard," given that's your stated target. Keep in mind, we're talking about people who aren't adventurers, so they aren't delving into murder-holes and sparring with bus-sized lizards with superiority complexes and epic halitosis.
How long does it take?
AND who says you live to get 12th level or higher? Depending on the editions they answer may vary. Leveling up in the world is very very dangerous.