ABC’s “Dancing With the Stars,” taking advantage of a repeat “Two and a Half Men” on CBS, dominated Monday’s primetime ratings race with its 12th-season premiere Monday despite falling off by more than 15% among young adults. The net’s “Castle” also led its hour with season highs.

According to preliminary nationals from Nielsen, the two-hour season premiere of “Dancing With the Stars” — with a cast that includes Kirstie Alley and Sugar Ray Leonard — was easily Monday’s top-rated program, averaging a 5.1 rating/15 share in adults 18-49 and 22.3 million viewers overall (with the demo score expected to rise to at least a 5.2 in the nationals). These are also the top same-night scores for any regular series this calendar year with the exception of Fox’s “American Idol.” Still, the show figures to come in about 17% below last spring’s especially strong opening night but within about 8% of that show’s overall audience.

Also for ABC, 10 p.m. drama “Castle” took control of the 10 o’clock hour (3.3/9 in 18-49, 12.7 million viewers overall), achieving season highs.

Elsewhere, CBS ran second even though it was down noticeably from last year when “Two and a Half Men” aired an original. The evening kicked off with a season low for “How I Met Your Mother” (2.9/9 in 18-49, 8.0 million viewers overall), and newcomer “Mad Love” fell off from there to a distant third-place finish at 8:30 (2.3/7 in 18-49, 6.5 million viewers overall). At 9, a repeat “Two and a Half Men” (2.6/7 in 18-49, 9.1 million viewers overall) settled for about half of what an original did opposite the “Dancing” premiere a year ago, and that led into lower than usual scores for “Mike and Molly” at 9:30 (3.0/8 in 18-49, 9.6 million viewers overall), although it did build on its “Men” repeat lead-in. And at 10, rookie drama “Hawaii Five-0” (2.9/8 in 18-49, 9.8 million viewers overall) was up vs. its most recent original to place a strong second in the 10 o’clock hour.

At Fox, “House” was on the low side but placed second in the 8 o’clock hour (3.4/10 in 18-49, 9.3 million viewers overall), followed by “The Chicago Code” (1.9/5 in 18-49, 6.3 million viewers overall), which was flat week to week and placed third in the 9 o’clock hour.

NBC’s struggling Monday dramas “Chuck” (1.5/4 in 18-49, 4.3 million viewers overall), “The Event” (1.2/3 in 18-49, 4.0 million viewers overall) and “Harry’s Law” (1.6/4 in 18-49, 8.2 million viewers overall) all matched or set series lows.

Preliminary 18-49 averages for the night: ABC, 4.5/13; CBS, 2.8/8; Fox, 2.6/7; Univision, 1.5/4; NBC, 1.4/4; CW, 0.2/1.

In total viewers: ABC, 19.1 million; CBS, 8.8 million; Fox, 7.8 million; NBC, 5.5 million; Univision, 4.1 million; CW, 0.7 million.