The percentage of first-time buyers who can afford a home in Los Angeles County has fallen again. According to statistics released by the California Association of Realtors, 19 percent of first-time buyers could afford a median-priced home in the county in the third quarter of 2006, down from 23 percent a year ago. The same pattern held true for the state, where 24 percent of first-time buyers can afford a home, down from 28 percent a year ago. The county median for single-family homes, $494,690, is higher than the state median of $478,710. In the San Fernando Valley, the median home price has risen to over $625,000. C.A.R. said that the minimum household income to buy a $478,710 home was $98,890, based on an adjustable interest rate of 6.58 and assuming a 10 percent down payment. First time home buyers typically purchase a home equal to 85 percent of the prevailing median price, the agency said.