Obviously it is not just a matter of definitions, albeit ageism and "one size fits all" approach is nonsense precisely during the so called "third" and "fourth" age. What the article seems to propose again is something similar to the " compression of morbidity ", originally introduced by north American gerontologist James Fries in the early '80s. Since then indeed strong evidence based arguments have been provided in favour of healthier life style for better ageing.
However we should be also aware that talking about "active ageing" and "successful ageing", as Canadian sociologist Stephen Katz argues, is also linked to a lifestyle idea that is connected to individualism and consumerism in contemporary societies and therefore should be always put into context (for example of current increasing social inequalities and therefore uneven conditions of access to services and resources).
In fact there might be also "evidence based" reasons in support for other more radical options.
Problems result from any attempt to describe the ageing population as a single entity because of the huge age range, from 65 to 105. No one would attempt to generalise from birth to 40, or 40 to 80, so why generalise about “the elderly”?