In this weekend's box office battle between youth and age, age pulled an upset victory. Newly-minted 18-year-old action star Alex Pettyfer was widely expected to take the weekend with aliens-on-the-run spectacle 'I Am Number Four,' but the victory went to 58-year-old serious-thespian-turned-action star Liam Neeson, whose 'Unknown' debuted at No. 1 with an estimated $25.6 million over the four-day weekend.

'Unknown' was successfully marketed as an international thriller similar to 'Taken,' Neeson's hit from this time two years ago. That movie enjoyed a three-day opening of $24.7 million, and 'Unknown' was in the same ballpark, with $21.8 million from Friday to Sunday. And Neeson's age-appeal was no small reason for 'Unknown's' success. Studio research indicated that a full 54 percent of the film's audience was over 50.
'Unknown's' biggest competition came from the opposite end of the age spectrum, as holdover 'Gnomeo & Juliet' took second place with an estimated $24.8 million, just $800,000 shy of 'Unknown's' take. The Disney toon was $2.4 million behind 'Unknown' on Sunday, but it enjoyed a late surge on Monday, with an audiences of kids who were out of school for Presidents' Day. In 11 days, the film has earned $55.8 million and seems well on its way to crossing the $100 million mark.

'I Am Number Four' had been expected to debut with as much as $30 million, based on its status as a bestselling teen novel, the presence in the cast of such youth-appeal stars as Pettyfer and 'Glee's' Dianna Agron, and IMAX surcharges on 228 out of 3,154 screens (the weekend's widest release). The filmmakers seemed to be trying to create another 'Twilight' style franchise (one where the powerful, persecuted boy is an alien, not a vampire), but 'Twlilight' was smart enough to take the point-of-view of the regular-girl female lead, while 'I Am Number Four' is shot from the boy's perspective. So the romance angle wasn't strong enough to appeal to girls (who were still drawn to 'Justin Bieber: Never Say Never' this weekend), while the sci-fi angle wasn't strong enough for boys. As a result, the film debuted in third place with an estimated $22.6 million over the four days.

Last week's winner, Adam Sandler's'Just Go With It' fell to fourth place with an estimated $21.7 million from Friday to Monday. Its three-day total was $18.5 million, down 39 percent from last weekend. Over 11 days, the romantic comedy has earned $64.3 million.

The weekend's other new wide release, 'Big Mommas: Like Father, Like Son,' debuted in fifth place with a disappointing $16.3 million from Friday to Sunday and $19.0 million in estimated sales over four days. Back in 2000, the first 'Big Momma's House' opened with $25.7 million over three days, and the 2006 sequel had a $27.7 million three-day weekend. It could be that, in the five years since, Martin Lawrence's star has dimmed, or fans outgrew the franchise, or maybe there just wasn't enough juice left in the fed-undercover-in-drag-and-a-fat-suit concept for a third movie.

'Unknown' - Trailer No. 1

The full top 10 (four-day totals):
1. 'Unknown,' $25.6 million (3,043 screens), new release
2. 'Gnomeo & Juliet,' $24.8 million (3,014), $55.8 million total
3. 'I Am Number Four,' $22.6 million (3,154), new release
4. 'Just Go With It,' $21.7 million (3,548), $64.3 million
5. 'Big Mommas: Like Father, Like Son,' $19.0 million (2,821), new release
6. 'Justin Bieber: Never Say Never,' $16.5 million (3,118), $51.4 million
7. 'The King's Speech,' $7.9 million (2,086), $104.7 million
8. 'The Roommate,' $4.5 million (2,160), $33.1 million
9. 'The Eagle,' $4.3 million (2,296), $15.8 million
10. 'No Strings Attached,' $3.7 million (1,966), $66.6 million

•Follow Gary Susman on Twitter @garysusman.
Based on 38 critics

An accident victim (Liam Neeson) awakes and finds that his wife does not recognize him. Read More

Big Mommas: Like Father, Like Son
Based on 14 critics

Malcolm Turner and his stepson go under cover at a girls school to find a killer. Read More

I Am Number Four
Based on 30 critics

An alien with extraordinary abilities poses as an ordinary human to evade those sent to kill him. Read More