When does ‘being old enough to know better’ start?

This a hard question to answer since the powers that be decided a Dementia patient is capable of being President and we are still living with that disaster currently.

You're old enough to know the answer to this question.

I’ll let you know when it kicks in, I’m still waiting.

Tomorrow

I guess it all depends on the situation right?

Your 30s.

You are gonna screw up left and right in your 20s. This is assuming you're living it up in your 20s - going out, partying, having a great time.

As long as you don't make any mistakes that will affect the rest of your life, NP - make the mistakes then. YOu've got plenty of time to recover.

By your 30s, you shouldn't be doing stupid things anymore.

It starts at 10 and gets progressively more ingrained in your life.

It was and probably still is a deliberately nebulous term so as to give high-handed parents maximum use of it, and in so doing, define it as broadly or narrowly as they please. :)