Common wisdom is not to do it until your channel is a certain magical size. Common wisdom is wrong, in my opinion. I say do it as soon as you'd like, even from your first video if you want to. The only video I do not monetize is the channel trailer.
Here's the reality: just about no one understands how advertising works on YouTube unless they're a creator. All most people know is that, as they're viewing YouTube, there is an advert every now and then. They do not associate that with the channel of the video they're watching.
Do you know how unlikely it is for someone to see one of your videos, notice that there's an advert, notice you're below a magical channel size and then get upset by it? Equally, do you know how many people are going to notice that your channel does not have adverts and then somehow reward you for that? Both numbers are so low that it's not worth worrying about.
For me, I view it as just another metric that I can see go up over time. When I see the 28 day trend go in the upwards direction, I view it as A Good Thing. The fact that it's a tiny amount of money is irrelevant to me; it's just another factor with which to judge success. I had it on for the first few weeks of my channel, then I turned it off for a few weeks, then I turned it on. Do you know what impact having it either off or on had on my growth? Zero.