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: 
The length of time you have established credit does improve your credit score in the long run.

As long as you can avoid paying interest, you might see if you can get a card with cash back rewards. I have one from Citi that sends me a $50 check every so often when I have enough rewards built up.
This is a good idea, but it will barely affect your credit score at all. 

Credit cards, while a good tool to use for giving a minor boost to your credit score and for purchasing things while also building up rewards with those purchases, aren't very good for building credit. 

This is because when banks calculate your credit report, they look at your *long-term* credit history, and weigh larger, longer-term debt much higher than short-term debt that you pay off right away. While having your credit card is better than nothing, it's a relatively small drop in the pond when it comes to credit. 

I would still recommend getting a credit card though - it will, if you haven't already started paying off a debt like a student or car loan, give you a credit identity and rewards depending on the credit card you choose. But if you do, **do not ever let yourself fall into delinquency**. Failing to pay off loans will damage your credit score. So if you do plan to get a credit card, it is much better to do as you've said and pay it all off as soon as possible. 

Edit: 

In addition to the above, using a credit card has the added benefit of having greater security over Debit cards, and ensures that your own money won't be stolen (though you will still have to report a fraudulent charge).