I'm fairly sure the '15 years' label is all to do with tax rates, any clothing for children under the age of 16 is subject to 0% vat/sales tax, nappies, even disposable ones are still classed as clothing and hence attract no tax.
Going back to the original post, the gap between baby nappies and adult incontinence products is always shrinking, depending what shop you where in there can be a seem less transition from nappy pants to pyjama pants to bladder weakness pants, some shops, these are all on the same aisle.
There often lined up in that order in my local Aldi, from size 7 nappies and pullups to age 4-7 night pants then the medium incontinence pants, Tesco is probably the best example with the junior and junior plus nappies entirely filling the gap between the baby and adults products.