Born on 7 June 1956 in Widnes, Lancashire, Sherwen was brought up in Kenya, where his father ran a factory which produced fertiliser, paint and insecticides. He started his sporting life as a swimmer, finishing second in the under-14 Kenyan swimming championship. Upon returning to Britain, he won the regional under-18 championship for Runcorn and District. However, at 16, he turned to cycling and rode for the Weaver Valley CC in Cheshire, receiving guidance from Manchester coach Harold Nelson, and trained regularly with other local riders destined for professional careers, notably Graham Jones, John Herety and Ian Binder.