When do Druids gain the ability to cast each spell level?

I understand spells and that spell levels don’t correspond with character level but on this website it says:

Spell level and character level don’t correspond directly. Typically, a character has to be at least 17th level, not 9th level, to cast a 9th–level spell.

I understand this, but what character level is a Druid required to be to cast a spell at any given spell level?

Just to be clear, I am not asking about spell slots here.