I have lived within 50 mins drive of LHR for the best part of 45 years, with LTN even closer, so have a lot of options. I agree that Eurostar is brilliant. However, to get from home, to local train station, then across to Eurostar, then an airport for a long haul... That is not worth the physical effort of suitcases to taxi, train, taxi, train, change trains to get to the airport.
Family can drive us to LHR in 45 mins, traffic dependent of course. I expect that most older folk will pay.