Learn Dutch: In mijn jeugd was ik altijd braaf. - In my youth, I was always well-behaved.

Because the sentence starts with the time phrase In mijn jeugd. In Dutch, whenever you start a sentence with something other than the subject, the verb and subject must swap places. This rule is called inversion.

Is was used for all singular subjects in the past tense?

Yes! The simple past tense of zijn (to be) is was for ik, jij/u, and hij/zij/het. You only change it to waren when the subject is plural, like wij (we) or jullie (you all).

Does In mijn jeugd mean the same thing as 'In my childhood'?

Yes, this is actually the most natural way to say 'in my childhood' or 'when I was young' in Dutch. The word jeugd covers your childhood and teenage years.

Can I use braaf to describe a good adult?

Usually, braaf is used for children and pets to mean 'well-behaved' or 'obedient'. If you call an adult braaf, it often sounds a bit mocking, like calling someone a 'goody-two-shoes' who never breaks the rules.