I'm inclined to agree with Danny. Both my and my other half's parents live in south London... they pretty much insist on Gatwick or Heathrow, they don't want a 6 am departure or 11 pm arrival, and they have a strong preference for a minicab to take them to/from the airport. Crowds and a cramped experience doesn't work for the over 70s. Wizzair's well-publicised propensity for cancelling at short notice is not suitable for those who are less than robust or going on an expensive holiday. Ryanair isn't suitable for over 70s unless travelling with younger relatives. People over 80 will often have valid concerns about travelling abroad with their state of health and what happens if they need medical treatment away from the family support network and outside the NHS
All of this will strongly discourage over 70s from south London from using the DART