Starfield has reportedly been in development for six years, or possibly more (thanks, TheGamer). The news isn't exactly new, though. Bethesda Artist Lucas Hardi posted a brief mention on his personal website about his involvement with Starfield, saying he's been working on it for six years and will share more information when he's able to do so. The date range is also on Hardi's LinkedIn page.
Reddit picked up the information over the weekend, but there's no indication when Hardi published this little tidbit or updated his LinkedIn page. If it is recent, Bethesda may be loosening some restrictions on information sharing, which could mean we'll be learning more about Starfield soon.
Bethesda Has Been Working on Starfield for At Least Six Years
Or it could just mean no one saw it until now. Either way, it seems active development, or at least work on Starfield's art, began soon after Fallout 4 released, though some rumors suggest Bethesda started pre-production as early as Skyrim's original release date in 2011.