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 guessed he was about three years old when he found them in 1989 and put down a date on his adoption papers indicating he was about two and a half. He celebrates his birthday on Earth in early May, 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, so he turns 19 in Oracle, Airdate: 05/04/06. Lois Lane makes him a special cake and gives him a journal. Shortly after, they break into Lionel Luthor's agenda, the on-screen indication that it is early May.
- Season 7: Apocalypse, Airdate 05/01/08. Lois Lane invites Clark out "for a brew" since his license says he's 21.
Kristin Kreuk was born December 30, 1982 meaning she began portraying Lana Lang when she was 18 years old. Lana was born in the Fall of 1986, making her about six months older than Clark. This would place her birthdate either in November or September.
- 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 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 probably early in the year. Her gravestone in Crusade lists her birth year as 1987. However, she turns 18 in Spell, which aired in November 2004, indicating that Spell actually takes place several months after its airdate, in early 2005.
- Season 2: Chloe is fifteen when she writes a love letter to Clark in Fever, Airdate 2/18/03. (She states this in Instinct.) This indicates her birthday is before this date.
- Season 4: Spell, Airdate: 11/10/04. Lois throws a rowdy 18th birthday party for her in Clark's barn.
- 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.
- 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. However, the year isn't visible on the birth certificate. November 12th was a monday in 1979 and 1984. As 1979 would've been too early, for the reasons stated above, 1984 would be the more likely candidate. This would then make Tess Mercer's date of birth November 12th, 1984.