I don't think that is exactly what he was saying. He was saying that the number of old people outnumbers the number of young people, thus creating a demographic bias. Even if the numbers are true – and that would depend on how you categorise old and young – there are also quite a lot of middle-aged people in between. I realise that he is talking about 16 and 17-years-olds who don't yet have a vote but who may have a vested interest in the future. I think he is very much mistaken in suggesting that old people don't concern themselves with the future and just vote for what suits them now. I have no idea how many old people are grandparents, but would guess there are quite a lot of us whose adult children's and grandchildren's futures are very dear to our hearts and whose future well-being matters to us more than our present circumstances.
I think he is also locked in a bit of a time warp. Many of those of us who are now getting older and grey-haired are the 16- and 17-year-olds of the 60s and 70s who were enthusiastic political activists. For example, many were "greens" early on. (Although we should never forget that our parents' generation used shopping bags, not plastic carriers, put out the "salvage" (a leftover term for old newspapers collected during the war, who turned collars on shirts and sheets sides to middle, who bought their children winter coats and school uniform several sizes too big because they would last three or four years as they grew, who cultivated vegetables on allotments, dutifully washed out their milk bottles for collection the next day and returned their beer and soft drink bottles to the shops where they were bought.)
Having been one who marched against America's war in Vietnam, who marched with CND, protested against animal testing of cosmetics and so on and so on. I lamented what I saw as a complete lack of interest in either local, regional or world politics in the next generation. Then George Bush and Tony Blair decided to invade Iraq and I was heartened by the number of young people who were angry enough to turn up early in the morning and stand around for hours before they could move on a cold February day to say "Don't attack Iraq". I especially liked the boys who shouted "Don't attack Iraq, innit."
I was horrified some years later when, on an election day, I asked a couple of young women who worked in the sandwich shop that we frequented while renovating a house if they had voted. They said that they never voted because "it was just for rich people".
Maybe, Professor Runciman you should get out of Cambridge once in a while and travel around the country. Talk to people of different ages, from different backgrounds, different histories and, just different from you.