So confused. How many years have passed?

It's been a while since I've seen season 4. They've taken forever to do the show and it's really distracting when all these kids are supposed to be like 14 and they are very clearly 20s. Also why didn't they cast someone new to play Holly? She was like 5 when the show started. Idk maybe I have my years wrong but everyone seems so old and there were definitely characters they could've recast if they were going to take this long to do the next season. Just a gripe not the end of the world but a bit annoying.