I don't see why demographics would be regarded as so important to industrialised societies. The bulk of labour power in those societies comes from machines, not physically fit young people. In such a society, an older population is not a downside, and can in fact be an upside, as older people often have more technical expertise for developing and maintaining those machines.

The only reason it's considered bad is because the system of socially-subsidised retirement leads to older people being artificially unproductive, but that's just one way of supporting the elderly; It would be feasible to institute a new system where aging people are given stronger labour protections, and are provided with support to change their patterns of work in response to changes to their physical and mental capabilities. Many would even see this as a positive thing, since work can provide a sense of meaning and retirement can be a major stressor.