debut episode gets 1.66 million viewers

Hey, thanks for sharing, I was curious about the ratings for the premiere.

Can any posters from the US tell me the significance of age ranges when compiling stats on viewing figures? I know it has something to do with advertising but what age range is considered to be the target group and why? Thanks.
From Deadline Hollywood:
Night 1 1.66 million viewers
Night 2 1.47 million
Night 3 1.35 million

Night 3 of CBS’ The Late Late Show With James Corden delivered 1.35 million viewers, according to preliminary stats, as well as a 0.5 ratings in adults 25-54, 0.3 in adults 18-49 and 0.2 in adults 18-34. That’s a decline of 310,000 viewers from James Corden’s unveiling as host of Late Late Show on Monday (1.66 million viewers). Wednesday’s early stats show he also hung on to his ratings with 18-34 and 25-54, and dipped 0.1 in 18-49.