I think part of the problem with your idea OP is that just mandatorily saying that everyone at 70 must retake their driving test potentially comes under the category of ageism, I think.
However asking, as someone else mentioned, for them to present at that stage evidence from their GP/Opticians that they are still fit to drive if perhaps an idea because there is medical justification due to the natural deterioration of age.
In UK law, age is a protected characteristic under the Equality Act 2010. However, age discrimination can sometimes be lawful if there is a "legitimate aim" and the measure is "proportionate". For example, there are already different rules at age 70. Driving licences must be renewed at 70 and every three years thereafter. Drivers must make a declaration that they remain medically fit to drive. Certain medical conditions must be reported to the DVLA.
So the principle of treating over-70s differently already exists. The question would be whether a compulsory practical retest is justified by the evidence.
Arguments against a retest: Many drivers in their 70s have decades of experience and very safe driving records. The data generally show that drivers in their early 70s are not among the highest-risk groups. A blanket retest could remove competent drivers from the road and reduce independence.
Arguments for a retest: Some age-related declines in eyesight, reaction time and cognitive function can occur without the individual recognising them. A practical assessment might identify people who are no longer safe to drive. Society already requires tests to gain a licence, so periodic reassessment is not an entirely new concept.
Personally, I think there is a stronger evidence-based case for what an earlier poster suggested. Regular eyesight certification from an optician. GP or medical practitioner confirmation where relevant conditions exist. Targeted driving assessments when concerns arise.
Rather than automatically requiring every 70-year-old to take a full driving test, regardless of their health or driving record.
In fact, one criticism of a GP sign-off system is that GPs often know little about a patient's actual driving ability, whereas eyesight can be measured objectively. Some experts therefore favour a mandatory eyesight tests at renewal. Disclosure of medical conditions and a specialised driving assessments only where there are concerns.
An interesting comparison is that we don't generally retest people because they're 17 to 24, even though that group statistically presents a much higher risk. If the sole objective were reducing insurance claims and collisions, a risk-based argument could be made for additional assessments of young drivers as well. That's one reason opponents of age-based retesting often describe it as unfairly focusing on age rather than demonstrated risk.
So I'd say: mandatory eyesight evidence at 70 would probably attract broad support, whereas mandatory retesting at 70 is much more likely to be viewed as potentially ageist unless there is strong evidence showing it improves safety enough to justify the different treatment.