The television network landscape has an upside-down look this fall: NBC, last among younger viewers for a decade, is suddenly on top.

Numerous unexpected factors have played a role in this surprising turnaround, some of which are not likely to last, but the numbers are clear. Among the viewers prized by most advertisers — 18- to 49-year-olds — NBC has beaten its network rivals every week of the new television season.

That’s three weeks, and NBC is in contention to post a fourth victory when the Nielsen accounting is official on Tuesday. (CBS, as it has for years, remains well ahead in terms of the total number of viewers.)

NBC’s ascent is the most striking development of the season, but it goes hand in hand with a wider story of network performance that, NBC aside, has ranged from disappointing to alarming.