How Old Every Main The Big Bang Theory Character Is (Beginning, End & During Young Sheldon)

Between The Big Bang Theory and Young Sheldon, Leonard, Penny, Sheldon, Raj, and all of the main characters in the franchise do a lot of growing up!
One of the over-arching themes of The Big Bang Theory and Young Sheldon is growing up and maturing as a person, and the ages of the main characters, including how old Sheldon Cooper is, reflect that. Over the span of 12 years for The Big Bang Theory and 6 for Young Sheldon that occurs both In-Universe and out, fans follow the major milestones of their favorite characters as they interact with their families, their schoolmates, change careers, get married and flourish both as individuals and a close-knit group.

Since The Big Bang Theory takes place in 2007 and Young Sheldon’s first season takes place in 1989, all the ages of the main characters can be inferred from these dates as well as clues from both series. From astrological signs to birthdays both mentioned and celebrated and references to various milestones in life, there are ways to tell exactly how old each main character from The Big Bang Theory and Young Sheldon is. Wherever necessary, some educated guesses have been made if nothing was explicitly stated from plot points or supplemental material from across the franchise.
8 Sheldon Is 27-39 In The Big Bang Theory (& 9-15 In Young Sheldon)
Sheldon Wrote His First Dissertation At The Age Of 15
Born February 26th, 1980, Sheldon is 27 in the pilot of The Big Bang Theory after having graduated high school at 11, college at 14, and received a PhD at 16. A prodigy, Sheldon has achieved more in life than most do in several yet social interactions elude him until he adapts to his friends’ quirks and enjoys a romantic relationship with Amy, ending the series at 39 winning the Nobel Prize along with her. When Young Sheldon begins in 1989 Sheldon is 9, and with the series’ 6th season over, he’s a teenager of 15, writing his first thesis.
7 Leonard Is 27-38 In The Big Bang Theory (& 11 In Young Sheldon)
Leonard Appears As A Young Boy In Young Sheldon Season 2
Like his roommate Sheldon, Leonard was already a professional scientist when most people would still be earning their PhDs. Given that Leonard was born on May 17th, 1980 and received his PhD at 24, that would make him 27 at the start of the series. Over time, he develops a greater sense of self-confidence, co-authors several scientific papers, marries Penny, and starts a family at 38 by the end of The Big Bang Theory. He appears in Young Sheldon Season 2 as a boy who, given the season took place in 1990, would have been 11 years old.
6 Penny Is 21-33 In The Big Bang Theory (& 5 In Young Sheldon)
Penny Is The Youngest Character In The Big Bang Cast
In the very first episode of The Big Bang Theory, Penny tells everyone that she’s a Sagittarius and didn’t graduate from her hometown community college. Later on, it’s revealed that Penny’s birthday is December 2nd, 1985, making her 21 years old and the youngest of the cast. Penny is 33 years old by the time the series ends, and she’s established a marriage of equals with Leonard after years of a will-they-won’t-they romance. Penny appears in the second season of Young Sheldon which takes place in 1990, meaning she would be about 5 years old.
5 Raj Is 25-37 (& 9 In Young Sheldon)

He’s The Only Character Who’s Still Single By The Big Bang Finale
Born on October 6th, 1981, Raj is the next youngest member of his group of nerdy friends after Penny, yet has still accomplished quite a lot. As the series progresses, the Cambridge University graduate ends up working as an astrophysicist and later with Sheldon so that his work visa doesn’t expire and is the only person to still be single at 37 when The Big Bang Theory wraps up. Just like Leonard and Penny, Raj appears briefly, like the rest of the main cast of The Big Bang Theory, in Young Sheldon’s second season and is about 9 years old.
4 Howard Is 25-37 In The Big Bang Theory (& 7 In Young Sheldon)
Howard Becomes One Of The Most Accomplished Big Bang Characters By The Finale
Given that Howard’s birthday year is determined to be 1981, he’s 25 (and just a little older than Raj) when The Big Bang Theory starts, though he has already been established in engineering with a Master’s degree. While he might begin as one of the series’ most lecherous characters, he ends the series married at 37 with two children after working on the Mars Rover and other secret government research projects. As with the rest of the main cast, he appears in the Season 2 finale of Young Sheldon in a non-speaking role at about 7 years of age.

3 Stuart Is 34-45 In The Big Bang Theory (Yet To Appear In Young Sheldon)
Stuart Is The Oldest Character In The Big Bang Cast
While he started as a recurring character on The Big Bang Theory, comic bookshop owner Stuart eventually became a main member of the cast halfway through the series. Stuart was born May 7th, 1974, making him 34 when the series began and the oldest of the group. He struggles to find meaning in his 30s until closer to the finale when he’s 45. Stuart begins dating his manager, and his comic bookshop earns some recognition. Stuart has not yet to appear in Young Sheldon alongside the other kids, but he will’ve been around the same age as Sheldon’s older brother.
2 Bernadette Is 23-33 In Big Bang Theory (& 6-7 In Young Sheldon)
Bernadette Became The Wealthiest Member Of The Big Bang Cast
Something that makes no sense about Bernadette on The Big Bang Theory is that she’s one of the few characters whose birthday isn’t mentioned. However, she was around 18 when she appeared in the Miss Quiznos 1999 beauty pageant, making Bernadette about 23 when she was a PhD student working at the Cheesecake Factory. After earning her PhD at 27, she eventually worked for ZanGen, a renowned pharmaceutical company that made her a wealthy woman by the series finale. She appears in the Season 2 finale of Young Sheldon as a child of 6.
1 Amy Is 30-39 In The Big Bang Theory (& 11-39 In Young Sheldon)
Her Voiceover Work Allowed Her To Appear In Young Sheldon As An Adult
Amy was introduced in The Big Bang Theory season 3 with a birthday of December 17th, 1979, making her about 30 at the time. She goes through the series as a well-respected and acknowledged neuroscientist who ultimately wins the Nobel Prize in physics as well as Sheldon’s heart. While Amy does appear in the Season 2 finale of Young Sheldon at about 11, she also has a voice-over cameo as an adult in Season 5, Episode 16, “A Suitcase Full of Cash and a Yellow Clown Car” which makes her about 40.