In the race for prize demographics between Hillsborough and Pinellas, it's not even close.Hillsborough County and Tampa residents are younger, they have more children and their communities are growing more quickly, exactly the type of statistics that please employers, excite urban planners and make businesses like the Tampa Bay Rays take note.Pinellas County, even counting St. Petersburg's downtown renaissance, is aging with fewer kids and married couples. Not surprisingly, population growth lags well behind the boom across the bay and the nation.According to five-year estimates released this month by the U.S. Census Bureau, the average number of Pinellas households dropped 4,805 between 2010 and 2014, a 1.2 percent decline from the prior five-year-period. By contrast, Hillsborough increased by an average of 21,708 households, a 4.8 percent increase — tops in Tampa Bay.And while Tampa Bay's 2.3 percent growth in households topped Florida's 2 percent, the region still trailed the national average (3.2 percent), another reminder of how much the Great Recession slowed state and regional growth.Why does the number of households, especially with children, matter? And who cares about the average age?Demographers say these numbers serve as indicators about the future of the labor force and the number of workers paying taxes, creating demand for good jobs and filling tax coffers.Hillsborough officials are understandably bullish about the numbers, but they do acknowledge a possible nasty underside: snarled traffic from far-flung subdivisions, for one."There's the old saying: Demography is destiny," said Jacob Cremer, a land planning lawyer at Stearns Weaver Miller, a Tampa law firm. "The normal narrative we hear is there are a lot of great things going on in downtown St. Pete, but that's sort of a thin market. You have young professionals and retirees. The bigger story is young families with school-age children moving into Hillsborough County."Pinellas planners, while acknowledging the county has a steep path to remain competitive, say it's misleading to compare county to county or Tampa to St. Petersburg.Why? It comes down to room to grow. Hillsborough has plenty of it, especially in the southern part of the county. Pinellas is pretty much built out."The basic problem is that we're out of undeveloped land so we have to redevelop everything," said Mike Meidel, Pinellas County's director of economic development. "It's a lot more work and a lot more expensive to make that happen."That disadvantage is reflected in the drop in construction. While the average number of housing units built fell by a staggering 60.6 percent in Hillsborough from 2010 to 2014 following the Great Recession, the drop was even worse in Pinellas, where the average number of housing units built declined by 70.7 percent, worse than the national average drop of 64.5 percent. To keep pace with their Hillsborough counterparts, developers in Pinellas, and especially St. Petersburg, have to tear down or expand aging houses on tiny lots to lure young families. That's more difficult than bulldozing another orchard or farm to make way for an entire subdivision."St. Pete is a mature, urban community," said Dave Goodwin, the city's planning and economic development director. "That's why anybody who improves their house (adds a bedroom and bath), those are important investments in this city."Many of St. Petersburg's homes were originally built for retirees who didn't need more than two small bedrooms and a bath. "The legacy of that housing stock is a challenge for us," Goodwin said. "You get to the point where you're having kids (and house hunting) and it's hard to find it in St. Pete. That's where we're less competitive."Housing has since recovered, so now Hillsborough County has a much different problem. Growth has spiked so quickly that the county has hired 19 employees to review site plans and inspect building sites. In booming south county, in the shadow of the new St. Joseph's Hospital and an Amazon fulfillment center, permit requests for residential construction have more than doubled."We had a lot of subdivision zoned before the recession hit and those opportunities are starting to become realities," said Lucia Garsys, the county's chief development and infrastructure services administrator. "We still have opportunity for ample residential development and that's what you're seeing in south county. It's a big market and that's because there are places to build."Where houses spring up, children usually follow. The statistics back it up. School enrollment dropped by an average of 6.4 percent, or 8,880 students, in Pinellas from 2010 to 2014, while Hillsborough was the mirror opposite: a 6.2 percent gain, or 14,049 students.The same pattern emerged for married households and households with children under 18: a drop in Pinellas; an increase in Hillsborough.But those numbers can tell an incomplete story. For instance, Pinellas has been good at creating jobs, said Meidel, the county economic development director. The county has a net loss in payroll, meaning people who work in Pinellas live elsewhere. And don't forget the much-touted downtown boom in St. Petersburg. That's being driven largely by single childless millennials and 50-something empty-nesters who are flocking to the apartments and condos dotting the city's growing skyline. That influx has been good for the city's image and tax receipts, but isn't as easily captured in census data, Meidel said.The question is: What happens when those millennials start to marry and have kids and the apartment starts to feel cramped?For incoming St. Petersburg council member Ed Montanari, the answer to that question could determine the city's long-term future."Where are they going to live, 10 to 15 years from now?," Montanari said. "Those are the kind of foundational issues that we need to address."Computer-assisted reporting specialist Connie Humburg contributed to this story. Contact Charlie Frago at [email protected] or (727) 893-8459. Follow @CharlieFrago. Contact Steve Contorno at [email protected] or (813) 226-3433. Follow @SContorno.