When you can conceivably afford to become one is the best answer I can come up with. I bought my first piece of real estate when I was 26. One rule I would follow is the 80/20 rule. If you can afford a $100,000 house, look for an $80,000 one. If you can afford a $200,000 house, look for a $160,000 one.
The 80/20 rule should really be applied to all finances in your life, and I'm a firm advocate of it, especially in light of what happened recently with the recession, and how much of that was as a result of too many people overextending themselves, and spending money they did not have.
The 80/20 rule basically says you only spend and live on 80% of your net income. I say net, because you never see gross. So, base it on your net. The other 20% goes in the bank, or in other investment vehicles such as the stock market (but ONLY IF you understand how the stock market works—don't gamble with your 20%). My suggestion, however is to DEFINITELY learn about it. Buy some books on it. Figure it out. The stock market has had its ups and downs, but it's still one the best percentage gainers your savings/portfolio will ever have.
It stands to reason that if you follow the 80/20 rule with your income, and extend that to your spending habits (buying 80% of the house or car you can afford to buy), it will compound the 80/20 rule, and down the road you will come to realize that you are living on less than 80% and putting away more than 20%.
Living your financial life in this way means you will be better prepared for dips and unexpected situations such as job losses, or recessions. You will require less credit. You will have more personal freedom, will likely not have to work as hard or as many hours. You will be in control of your life and your finances. And you will probably be able to absolutely consider early retirement as a viable option.
People think that saving means you will have to sacrifice. In reality, the opposite is true. The less you save, the more you have to earn, and the longer you will have to work. It means, you will have to sacrifice MORE the less you save.
Too many older people today have to keep on working because they're not done paying for stuff. Keep that in mind as you're young enough to be in the best position to get on the right path NOW. :)