My 13-year-old has an Oyster card that lets her travel free on buses, and it saves us a great deal of trouble and worry. If she's playing football in Catford, for example, or wants to go to Forest Hill Library, she can jump on a bus with a friend and get home safely and quickly. No loose change, etc. or long walks down a major road making herself a target.
She had to sign a code of behaviour promising not to use bad language or play loud music under threat of losing her card; maybe we should try getting that one enforced!