Question: I spend about $20-30 a week on my debit card, with periodic $200-300 purchases every few months. I've thought about getting a credit card the moment I turn 18 and using it to buy everything, then immediately paying it off. Assuming I only use it to buy things I can afford (which I trust myself to do), essentially treating it as a debit card, is this a good idea? Or should I wait until I'm a bit older? 

Assume that I will only spend money I have in my bank account at the time I spend it and that I will keep track of how much I spent and never spend more than I have.

Joe Taxpayer recommended I post an update: I ended up getting a card, and from my perspective the only thing that's changed is that it takes things ~24 hours to show up on my statement and I don't have to enter a PIN when I checkout. Should I add this to the question? I know on other SE sites people generally don't like updates in the questions

Answer: 
I want to recommend an exercise:

Find all the people nearby who you can talk to in less than 24 hours about credit cards: Your family, whoever lives with you, and friends.

Now, ask each of them "what's the worst situation you've gotten yourself into with a credit card?"

Personally, the ratio of people who I asked who had credit cards to the ratio of people *with horror stories* about how credit cards screwed up their credit was **nearly 1:1**. Pretty much, only one of them had managed to avoid the trap that credit cards created (that sole exception had worked for the government at a high paying job and was now retired with adult children and a lucrative pension). Because it's trivially easy over-extend yourself, as a result of how credit cards work (if you had the cash at any second, you would have no need for the credit).

But do your own straw poll, and then see what the experience of people around you has been. And if there's a lot more bad than good out there, then ask yourself "am I somehow more fiscally responsible than all of these people?".
Definitely not. Credit cards only exist to suck you into the soulless corporate system. What you want to remember here is that you can't trust banks, so you'll want to convert all your savings into some durable asset, say, bitcoins for example, and then hoard them like Smaug until after the Fall.