What classes are considered to have required at least some training (other than learning by themselves) in their past?

I'm mainly thinking of wizard, druid, monk, cleric and paladin, maybe bard too. Can we give our characters with those 'trades' backgrounds where they learned to do their thing on their own, or do they all need some sort of education, mentor, or something?

In the PHB, classes fall into three categories when it comes to the minimum age a character must be for that class:

Simple: Barbarian, Rogue, Sorcerer

Intermediate: Bard, Fighter, Paladin, Ranger

Advanced: Cleric, Druid, Monk, Wizard

But that having been said, the modifiers don't make a huge difference necessarily.

For humans for example, the base adulthood age is 15 years. Then for simple classes you add 1d4, 1d6 for Intermediate and 2d6 for Advanced. This means the starting age for a level one basic class human is 16-19, 16-21 for intermediate and 17-27 for the advanced classes.

I always took it to indicate the learning required for those classes, but this could easily be self taught knowledge in my opinion - as long as you had access to the necessary materials, arcane textbooks for example as a wizard.