Right now, The Big Bang Theory and its prequel Young Sheldon are in quite a unique position. As the former series heads into Season 12 and the latter series heads into Season 2, the writers have been able to create characters for the prequel and then introduce them on the present day-set show. During the Big Bang Theory writers panel at Comic-Con 2018, the writing team, actors Kunal Nayyar and Mayim Bialik, along with showrunner Steve Holland and former showrunner Steve Molaro were present, and they talked to audiences about how at least one special Young Sheldon crossover is planned for Season 12. Per Steve Holland:
There's an episode we're writing now where we're going to be introducing Tam, who is his best friend on Young Sheldon. We're going to learn what happened to them and why Sheldon's never mentioned Tam in the 11 years he's been on Big Bang. It's going to be a bit of a mystery.
If you've seen more of The Big Bang Theory than Young Sheldon, Tam is Sheldon's friend from middle school. He's a character that was introduced into the shared universe through Young Sheldon during its first season, and because of that, we've never heard hide nor hair from Tam on the original series. It's a little strange, because in Young Sheldon, Tam is Sheldon's only friend outside of his relationships with his family.
During the panel, the writers also mentioned that there haven't really been two major television shows where the writers can write to the characters in "different stages of their life." There are, of course, shows like Fear The Walking Dead and The Walking Dead that have prequels with some minor intersecting characters. There is also Better Call Saul and Breaking Bad, but the former only hit the airwaves after the latter was finished in that case. As Young Sheldon and The Big Bang Theory are running simultaneously, there should be plenty of opportunities moving forward to criss-cross and interplay in ways that other shows can't.
At the end of Season 11, when Sheldon and Amy finally tied the knot, we were able to spend time with the adult version of Sheldon's brother, who is played by Jerry O'Connell. Jerry O'Connell, who moderated the panel, also said of the experience:
It was really great to play George, because I did my research watching Young Sheldon, mimicking Montana Jordan's accent. I mean, I had 20 episodes to study... [My kids], they watched Young Sheldon, and then I explained to them I was working on the original young Sheldon. They were like, 'What?' Young Sheldon's the original. I had to be like, 'No, I'm working on the OG Young Sheldon.'