Common misconceptions here. If have a credit card with a limit of $10,000 and you pay it down to 0 every months, you will end with a super high score. Why? Because the credit card company will report that you are making an on time payment every month and that you have $10,000 of credit and are using hardly any, thus implying your income is sufficient for you to avoid debt. What kills your score is keeping a balance of $9500 on that card and only paying the minimum. This implies you are living beyond your income. The lower the % of credit that you use VS the amount you have available pushes your score up.
If you truly have no credit card, car loan or mortgage you will end up with a 0 score. A 0 score does NOT equal bad credit and you can get a mortgage with a 0 score as long as you have a down payment, a stable life and money in the bank.
Bad credit scores of 400 to 620 usually mean judgements, delinquent credit cards, over limit credit cards, bankruptcies and collection accounts. These things are what lenders look at NOT just the score.