Paying off your loan early is great but when it comes to building credit... its not so much about crediter getting their interest, if you pay off a loan immediately it doesn't really show steady paying ability. Having a track record is key for credit history.
I tell young people with no credit to grab a credit card and a small loan. Payoff the credit card every month and pay at least a year on the installment loan. (most people have a small car loan anyhow) Just make sure you have a job! Thats how I built my credit.
