'X Factor' Teens, Over 25s Fight To Impress Justin Bieber, Will.i.am.

It was a battle of, and for, the ages on Thursday night's "X Factor," as the oldest and youngest contestants on this year's show fought to secure a spot in the live shows. Sitting in judgment were guest mentors Justin Bieber and Will.i.am, who both copped to the difficulty of choosing between six admittedly strong competitors.

To say that L.A. Reid was not enthused about mentoring the over 25s would be an understatement. "I really didn't want this category," the music industry legend said bluntly, reiterating his disappointment in drawing what he considered the long (in the tooth) straw among the judges.

Because he's known for nurturing young artists, he decided to bring one of those protégés, Bieber (as well as the Biebs' youthful manager, Scooter Braun) on board to help him with his group. "I'm not happy, but I still plan on winning," he told his charges, giving them his version of a pep talk by issuing the challenge to the singers to "prove to me that you're worth my time," since he's had a "change of heart" and whatnot. Yeesh.

Big boys (and girls) do cry
Reid had a grin of satisfaction when he watched man diva Jason Brock crush Fergie's "Big Girls Don't Cry." Bieber thought Brock didn't hold back vocally, but wondered whether the tech supporter had the whole package.

Tatted-up musician David Correy also went the lady singer route, emoting all over Jessie J's "Domino," scaling the heights with notes that gave Reid chill bumps. "I thought he was great ... he has a look to him," Bieber added. While Reid wasn't sure Correy is viable, Braun felt the pure passion and had no doubts.

Daryl Black put some R. Kelly soul on Hall & Oates' "She's Gone," for a performance that Braun said could make the professional musician a star with the right band behind him. "I didn't find the voice tone to be unique," Reid argued. The lone country singer in the crew, road working dad Tate Stevens threw a curveball with a twangy take on Brian McKnight's "Back at One" that he absolutely made his own. Reid liked the song choice and Bieber dubbed him charismatic, praising Stevens' voice, but wasn't sure if he was worth the $5 million prize.

The oldest fella in the group, face inked rough customer musician Vino Alan, 40, telegraphed his years of hard life through a gritty, dark cover of Pink's "Sober." Bieber felt it and Reid was loving the soul, but he wasn't sure if Alan had "the package" that could sell records.

The lone female in the group, overly confident vocal coach Tara "I've always been a star" Simon went rogue, whisper singing Hoobastank's "The Reason" until she started busting out her usual shouty, irritating vocal gymnastics. "She's got a big voice but she should be sure to have more of those more sweet moments with her voice," said a seemingly not blown-away Bieber.

Britney Spears got her dream category with the teens and her decision was perhaps the hardest one of the night, as all six brought game well beyond their years.

Confessions of a (wannabe) teen idol
One of the most promising of the bunch, Diamond White, did her mini R&B diva thing on a piano ballad version of Avril Lavigne's "I'm With You" that had plenty of emotion. Brit's guest mentor Will.i.am. loved the voice, but worried that White was too hung up on her stage moves, with Britney agreeing that White needed help with stagecraft.

The key for Reed Deming was to get over his stage fright, which he seemed to do with Plain White T's "Hey There Delilah," confidently moving around and showing a variety of shades in his vocals. Spears still sensed some nerves and thought Deming could do better.

There were no nerves for Ohio rapper James Tanner, but his corny, vanilla take on the Shop Boyz's "Party Like a Rockstar" quickly devolved from embarrassing to mortifying. Britney dubbed it "entertaining," but said it was probably not strong enough.

Second timer Arin Ray had no such problems, as the Cincinnati kid killed his feathery, soul-kissed run through Nicki Minaj's "Starships." Spears agreed with Will that Arin needed work, but said he was worth taking a chance on.

Before taking the stage, 13-year-old Beatrice Miller was overcome by emotion, but got it together for an inspired, powerful acoustic version of David Guetta's "Titanium." "That was fresh," Will declared, though Spears agreed thought Miller needed to get some control over her vocals if she is to advance to the live shows.

Carly Rose Sonenclar seemed to have dialed it back too much on Karmin's "Brokenhearted," but proved her preternatural vocal prowess by the end. Spears and will deemed her "possessed," though Britney questioned her ability to cope with the pressure.

Next week's shows will reveal who makes it to the live finals.

Who do you think made the cut on this week's shows? Let us know in comments below.