Ratings for CBS’s “Survivor: Exile Island” have been eroding all season, and with the final episode just days away, they are not getting any better.

Last night’s penultimate episode averaged a 5.7 overnight rating among adults 18-49. While that was well ahead of the second-place finisher, NBC’s sitcoms, it was down 17 percent from the same night last year, when the second-to-last episode earned a 6.9.

In addition, it was off 0.1 rating point from the previous week. That suggests that Sunday’s season finale, which will air opposite ABC’s highly rated “Extreme Makeover: Home Edition” and “Desperate Housewives,” could be much lower than last year. Last year’s spring finale averaged a 7.6, but even that was down 24 percent from the previous year.

“Survivor” may simply be showing natural signs of age. The reality show is in its 10th season, and many reality programs don’t make it past three or four.

It’s possible that CBS, in anticipation of greater falloff, will move “Survivor” to a different night this fall, but that seems like a drastic move despite the show’s declines. It’s still a top program among 18-49s, and as long as the competition remains weak at 8 p.m., even a weakened “Survivor” will continue to dominate.

Meanwhile, CBS won the night among 18-49s with a 6.6 rating and 18 share. NBC was second with a 4.4/12, Fox third with a 3.6/10, ABC fourth with a 2.3/6, WB fifth with a 1.6/4, Univision sixth with a 1.5/4 and UPN seventh with a 0.9/3.
 
CBS finished first every hour, starting at 8 p.m. with a 5.7 for “Survivor” followed by NBC with a 3.7 for “Will & Grace” (3.4) and “Earl” (3.9). Estimates for NBC’s three shows from 8 to 10 p.m. are approximate, as they were supersized to 40 minutes each.

Fox was third with a 3.3 for “That 70s Show” (3.0) and the beginning of “That 70s Show Retrospective” (3.5). WB was fourth with a 2.0 for “Smallville,” ABC fifth with a 1.6 for “American Inventor Countdown to the Finale,” Univision sixth with a 1.5 for “La Fea Mas Bella” and UPN seventh with a 1.1 for the finale of “Everybody Hates Chris” (1.3) and “Love, Inc.” (0.8).
 
At 9 p.m., CBS led with an 8.2 for “CSI,” followed by NBC with a 4.0 for “My Name is Earl” (4.3) and “The Office” (3.8), and Fox with a 3.9 for “That 70s Show Retrospective.” ABC was fourth with a 2.6 for the finale of “Inventor,” Univision fifth with a 1.7 for “Barrera de Amor,” WB sixth with a 1.1 for a repeat of “Smallville” and UPN seventh with a 0.8 for “Eve” (0.8) and “Cuts” (0.8).
 
At 10 p.m., CBS completed its sweep with a 5.8 for “Without a Trace.” NBC was second with a 5.5 for “ER,” ABC third with a 2.8 for “Primetime,” and Univision fourth with a 1.3 for “Aqui y Ahora.”
 
CBS also led the night among households with a 13.2 rating and 21 share. NBC was a distant second with a 6.7/11, Fox third with a 4.8/8, ABC fourth with a 4.2/7, WB fifth with a 2.3/4, Univision sixth with a 1.9/3 and UPN seventh with a 1.5/2.