Better start now man! Take it from someone who knows.
<sarcasm>I'm 28, and had no clue what Credit was until about a year and a half ago.</sarcasm>
I decided I wanna buy a House someday, so I'd better build some Credit.
My brother used to be a Credit and Loan Officer for a bank locally, and in Utah as well. He told me the best ways to do it were to - If you have insufficient Credit, (like I did) Get a Secured Credit Card. It's a Start, (the Interest is borderline sodomy!). If you can't get that, then get a Gas Card from Mobil, or one of those places. That was where I started about 2 years ago.
I've had a Checking account since I was 18, but my brother says that they really tend to overlook most checking acct activity on Credit Reports. Charge it up, but pay it off ASAP. I did it for about a year, my next card was a Best Buy Credit Card, I was approved for a $1500 Credit Line. I Charged it almost full though, but I've half paid it off in 4 months!
After getting the Best Buy card I got a pretty Low Interest Capital One (7.9% Average, 9.6% Late/Overdue) Which isn't too bad for a guy w/ No credit!
I also got a Cell Phone earlier this year, for nothing, basically. Went to Cingular, They ran a Credit Check on me, and wanted nothing up front. Pretty Cool.
Now that my credit is slowly building, I see more crap come in the mail, more pre-approved gold cards, etc.......
Just be careful, and read the fine print, 'cause some of those credit cards are worse than Loansharks on the Street Corner!
Like I say, I wanna buy a house someday, and u need credit to do it!
My next thing is to take out a Loan, and Pay it off Sooner, than Later.