Older forms of English called children “babies” and “infants” until they were 6 or 7ish years old. If you read old accounts and diaries from 300 years ago, it’s weird to have them talking about a baby or infant and suddenly they mention that it’s actually a 6 year old. I’m guessing they do something similar in England, although maybe not to as old of an age.
Grade schools in Britain are still called ‘infant school’.