Tom Welling was born April 26, 1977, making him ten years older than Clark Kent. Clark Kent's birth date on Krypton is unknown, but his adoptive parents put down a date on his adoption papers indicating he was about two and a half. He celebrates his birthday on Earth May 3, 1987.
- Season 2: Calling, Airdate: 05/13/03. His age is not given but it is assumed that he is turning 16. Lana brings him a birthday cake and they share their first kiss in his loft.
- Season 3: Talisman, Airdate: 05/20/04, Age 17. He misses his birthday party when he is injured in an attack.
- Season 4: Run At the start of his senior year, Jonathan states that he is 17, so he must have turned 17 in Talisman.
- Season 5: He is stated to be 18 in Hidden and Aqua (with these episodes being set in 2005, it places his official year of birth as 1987), so he turns 19 in Oracle, Airdate: 05/04/06. Lois Lane makes him a special cake and gives him a journal. The next day, Clark and Chloe break into Lionel Luthor's agenda, the screen shows that Lionel has a meeting May 5, which they identify as tomorrow. This means that it's currently May 4, with Clark having celebrated his birthday the evening before. Placing his birthday celebration on May 3.
- Season 7: Apocalypse, Airdate 05/01/08. Lois Lane invites Clark out "for a brew" since his license says he's 21.
Erica Durance was born June 21, 1978. In Gone (2004), Lois says that her mother had passed away 15 year ago, when Lois had been six. This would've placed her birth in 1983. However, she was later portrayed as a college freshman, when her cousin, Chloe (born 1987), is entering her senior year of high school. This would make 1986 as her more likely year of birth.
- Season 1: Craving, Airdate: 11/27/01. Aunt Nell plans an elaborate celebration at the Luthor Mansion. Clark misses the party, but tries to make it up to her by recreating a childhood memory of a drive-in theater.
- Season 2: Redux, Airdate: 10/29/02. Lana finds a photo of her mom and Henry Small, dated July, 1985. Which Lana identifies as being less than a year, before she was born.
- Season 4: Crusade, Airdate: 09/22/04. Lana receives the Mark of Transference in Paris the day before her 18th birthday.
- Season 8: Bulletproof, Airdate: 01/22/09. Clark Kent when he sees Lana behind the counter at the Talon says "If I squint hard enough it is like we are 14 again" indicating that she was 14 years old just like Clark Kent when Lana started working at the Talon during the first season of Smallville, contradicting what was established in Crusade.
Michael Rosenbaum was born July 11, 1972. In the Pilot, it is established that Lex Luthor was nine years old the day of the meteor shower, so he was probably born in 1980, making him 21 years old when his father banishes him to Smallville to run the fertilizer plant.
- 10th birthday: His father gave him a copy of The Will to Power (Reaper)
- 12th birthday: His father gave him a lead box, but no one attended his party. (Memoria)
- 14th birthday: His father gave him a copy of The Art of War. (Fanatic)
- 23rd birthday: His friends at the Talon throw him a surprise birthday party (Chimera, Part 2) This comic book story takes place directly after "Suspect", airdate: 01/28/03, possibly placing Lex's birthday in January or February.
Allison Mack was born July 29, 1982. She is five years older than Chloe Sullivan. Chloe's birthday is sometime early in the year, with her year of birth being 1987.
- Season 2: Chloe is fifteen when she writes a love letter to Clark in Fever, Airdate 2/18/03. (She states this in Instinct.)
- Season 4: Spell, Airdate: 11/10/04. Lois throws a rowdy 18th birthday party for her in Clark's barn. While the episode aired in November, the previous episode had seen the end of the football season (late December). This means that the episode takes place in early 2005. Which would line up with her gravestone in Crusade listing her year of birth as 1987.
- Season 8 Chloe celebrates her 22nd birthday in Hex, Airdate 3/26/09, at least five weeks after her November 2008 wedding (stated in Turbulence.) Thus, it is likely that Hex takes place in early 2009.
John Schneider was born April 8, 1960. He is six years younger than Jonathan Kent. His gravestone in "Homecoming" lists his birthdate as September 1, 1954. This is in contradiction to "Relic" though, as that episode shows Jessica Kent explicitly pregnant with Jonathan in 1961.
Annette O'Toole was born April 1, 1952. Martha Kent's date of birth was never established. However, as Nicodemus establishes that she and Jonathan took the same class at Metropolis University, it's likely that Martha was also born in 1961.
- Season 8: An article on Tess, in "Turbulence" (Airdate: 03/19/09), give her then current age as 30. Which would mean that she was born between 1978 and 1979. However, as she's later revealed to be the younger half-sister of Lex Luthor (born 1980) and that Tess's mother was his nanny, those years are unlikely to be her year of birth.
- Season 10: Her birth certificate in "Abandoned" (Airdate: 11/12/10) lists her birthday as Monday, November 12th. The year isn't visible on the birth certificate. However, in the episode Tess and Clark looks at a photo of Tess and her adoptive parents, which is said to have been taken 20 years ago (1990). Tess is also said to have been five, when she was adopted by the Mercer family. Provided that she was adopted before November 12, she would've turned five in 1989 and been born in 1984. This would then make Tess Mercer's date of birth November 12th, 1984 (which lines up with the claim that she was born on a Monday).
- Season 6: Oliver is shown to have been a classmate of Lex Luthor. This would likely place his year of birth as 1980, the same as Lex.
Eric Johnson was born August 7, 1979. Whitney's date of birth was never established. However, the character's portrayed as a senior in Season 1. In "Kinetic", it's said that Whitney had lost his football scholarship. As he graduated in 2002, this would place his year of birth as 1984.
Jensen Ackles was born March 1, 1978. Jason's age and year of birth was never established. In Façade, Jason is said to be a college student, who had done his freshman year at Metropolis University. This would make him between 19 and 21-years-old, in 2004. Placing his birth between 1983 and 1985. A flashback in Veritas, shows Jason playing with Lex (born ca. 1980), Oliver Queen and Patricia Swann (born 1982) in 1989.
Sam Witwer was born October 20, 1977. According to Davis Bloome's adoption papers, his legal birthdate is October 16, 1987 ("Bloodline").