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 now in its 6th season, he's a teenager of 15, deep into his first dissertation.
Like his roommate Sheldon, Leonard was already a professional scientist when most people would be still earning their PhDs. Given that he was born on May 17th, 1980 and received his PhD at 24, that would make him 27 at the start of the series. Over the course of the series, he develops a much greater sense of self-confidence, co-authors several ground-breaking scientific papers, gets married to Penny, and even 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.
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, and later on it's revealed that her birthday was 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.
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 bit for his age. 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.
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 already been entrenched in engineering with a Masters 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.
While he might have started out 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. He was born May 7th, 1974, making him 34 when the series began and the oldest of the group, and he struggles to find his bearings in his 30s until closer to the finale when he's 45, he begins dating his manager, and his comic bookshop earns some recognition. Stuart has yet to appear in Young Sheldon alongside the other kids, when he wouldmost likely have been a teenager, around the same age as Sheldon's older brother.
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, but she was around 18 when she appeared in the Miss Quiznos 1999 beauty pageant, making her about 23 when she's 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 time she was 33 and the series ended. Like the rest of The Big Bang Theory's main cast, she appears in the Season 2 finale of Young Sheldon as a child of 6.
Like Bernadette, Amy was introduced in the third season of The Big Bang Theory with a birthday of December 17th, 1979, making her about 30 at the time. She goes through the series as a well-respected and revered neuroscientist who ultimately wins the Nobel Prize in physics as well as Sheldon's heart, and the two end up with one of the series' most special relationships by the time it ends and she's about 40. 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.
