At the age of 10 I was going out with my mates on our bikes for long days (from say 9am to 7pm), armed with packed lunches. We'd cycle up to 60 miles in a day, with much of it on busy main roads. By the age of 12 I was travelling all around East Anglia, and down to London, by train.
Later, my friend's son was happily travelling on his own, several times per year, between his home in Sheffield and his relatives' house in rural France by the age of 12. (Train to London, underground to Heathrow, flight to Paris, Metro into central Paris and a train from there).