There are the obvious signs – like lights and decorations everywhere, hearing Slade boom every time you enter a shop, and the Prime Minister suddenly taking an interest in religion – but there are more subtle, insidious ways in which Christmas creeps up on you. You know it's Christmas when:

You have pleasant conversations with London taxi drivers, who, instead of complaining about immigration and telling you how brilliant it was that David Cameron told the rest of Europe where to shove it, ask you what you're doing over Christmas, or remark on how satisfyingly quiet the roads are this time of year. Or even, as one cabbie did yesterday when I told him I was a journalist, recalled how he'd once been a toy boy for the legendary columnist Jean Rook. Such experiences can almost make you feel a seasonal warmth towards taxi drivers. Until, that is, you see the meter.

You suddenly feel very old when attempting to buy a present for a young person. This involves going to the sort of shop frequented by said young person. Yesterday, in an effort to get a gift voucher for my god-daughter, I went to a store called Abercrombie & Fitch. I could not have felt more ancient, or more out-of-place, had I arrived at an all-night rave wearing a tweed suit and cravat. In fact, I hadn't realised that, in popular shops like this, the desired effect is to create the exact atmosphere of a nightclub. They're dark, the music is pumping and, in this particular case, there are half-naked men strutting their stuff. I took one look at this hellish scene, walked out and resolved to present my god-daughter with the gift that every teenager seems to want this year: cash.

You feel completely out of the loop when young relatives hand you their Christmas lists. For instance, what on earth are FitFlop Happy Gogh clogs? Apparently, they're the shoes that anyone under 20 has to be seen in, and, so I've learned, can be worn barefoot when it's warm, or with stripy socks when it's not. Get with the beat, daddy-o.

You suddenly have an uncontrollable urge to buy the sort of confectionery you'd never think of having in the house, and in the sort of quantities that would be more suited to a family of junk food addicts in Arkansas. A tub of Celebrations the size of a bass drum? Of course. An industrial-sized tin of Quality Street? Don't mind if I do. And nuts? What's behind the seasonally-adjusted passion for walnuts that lasts until we throw them out at the same time as the tree?

You switch on the Today programme on Radio 4, and instead of an earnest discussion about the eurozone, you find a light-hearted item designed to make us feel the world is not such a serious place. Yesterday, for instance, it was on how one's surname sometimes reflects one's profession – like Doctor Death or Reverend Vickers. Hilarious. By the way, this column has been brought to you today by Simon Curmudgeonly.