I think it used to be normal to use potties/toilets etc much younger - like 3-6 months and onwards - so by the time toddlers were walking they were often out of nappies in the day time at least. Now we have all sorts of different nappies marketed for different life stages (up to pullups and disposable pjs, for eg), which certainly help to prolong nappy use, so maybe there's some truth to it. It's about lifestyle factors as much as anything. If mum's a SAHM and sick of boiling nappies (as they used to do) they're probably a bit more motivated to get their babies out of nappies earlier.