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: 
Yes, it is a very good idea to start your credit history early. It sounds like you have a good understanding of the appropriate use of credit, as a substitute for cash rather than a supplement to income. As long as you keep your expenses under control and pay off your card each month, I see no problems with the idea.

Try to find a card with no annual fees, a low interest rate if possible (which will be difficult at your age), and with some form of rewards such as cash back. Look for a reputable issuing bank, and keep the account open even after you get a new card down the road. Your credit score is positively correlated with having an account open for a long time, having a good credit usage to credit limit ratio, and having accounts in good standing and paid on time.
> 
> 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?
> 
> 
> 

This is definitely a good idea.

From my own experience, before I got my first credit card through my local bank (age 18), I tired to apply for a card that has cash back rewards and was rejected because I didn't have any credit history. After I had the card from my bank for 6 months, I applied for [this Capital One](http://www.capitalone.com/credit-cards/quicksilver/) card that I've had ever since.