Rather than age I think I would be more interested in attitude and its impact on productivity. Some of the issues such as punctuality, being results-orientated, interested in learning new skills, adapting to new work practices, being able to work early in the morning and late into the evening if needed, following instructions without ego getting in the way, honesty, integrity, enthusiasm, polite customer service, ability to network and secure new revenue for the business from potential customers due to experience, knowledge and good inter-personal skills are key areas that do not depend on age.
The more that productivity is dependent on technology the more useful it would be if a recruitment process includes a test to check aptitude in being able to pick up and adapt to new technology. Not sure if testing for this in a recruitment process would be affected by Equality Legislation regarding disability? Can it be justified as necessary to raise business productivity?
Is productivity impacted by health issues - not sure what the stats are on this but it seems plausible? If productivity is affected by health issues e.g. being over-weight leading to multiple health and fitness problems. Would employers be taken to court for not hiring someone because they move slowly or get short of breath quickly or seem tired because they are unable to sleep properly?
UK workforce seem to have a problem for a while (or at least since the 2008 financial crisis) with wages not rising fast enough compared to increased costs of living because productivity of businesses is less than the productivity of competitors. Workers in the UK end up working longer hours for less pay.
"UK productivity growth has lagged behind that of other comparable economies since the 1970s and the country has suffered virtually zero growth in labour productivity since 2008"
https://theconversation.com/how-to-boost-uk-productivity-after-coronavirus-133735In terms of the issue of age though this article says "Worryingly for the future, the UK is the only country where older people (aged 55-64) outperformed younger people (16-24) in both literary and numeracy."