Rowan gives up school at 16 (Falconer's Lure). She drives up to school when she is in fact too young to do so (End of Term). Ready Made Family occurs the following Easter, the Cricket Term the following Summer, Attic Term the Autum after that, and Run Away Home that Christmas. At best Rowan is (barely) eighteen at the time of Run Away Home. Kay goes up to Oxford (eighteen) at the start of End of Term. She marries Edwin the following Easter (nineteen). By the next autumn (Run Away Home) she may (or may not) be twenty. If Giles is 22 it's the most he can be, and his rank is sub-lieutenant.