In what amounts to late middle age in television the CBS comedy "How I Met Your Mother" suddenly finds itself a hit. Seven years into its run this well-regarded series, which returns with a new episode on Monday night, is scoring its best ratings. It is up almost 20 percent this season among the audience that advertisers most want to reach, viewers between the ages of 18 and 49, and now ranks as the No. 8 entertainment show in that category. Beyond giving comfort to late bloomers everywhere, the performance by "How I Met Your Mother" is proving that the...