I opened the door this evening to someone who could have been our neighbour, or maybe someone who was looking for our neighbour. There is a farming “look” among the men in their 40s round here - skinny, almost undernourished, tough, weathered, unsmiling, so I find it difficult to tell one from another. In fact I have met him before - he is the councillor responsible for road repairs in our commune and he’d come to set up a meeting with us and the mayor to talk about the chemin rural that comes down to our property that we’ve been asking them to repair for 4 (or is it 5) years now. Given how important this is for us I think he was a bit disappointed I didn’t recognise him.
I needed a polite phrase along the lines of: I’m so sorry, I’m having a senior moment.
Any suggestions please as to what I should have said?
Je ne suis vraiment pas physionomiste et en plus à Mon Age ah là là (here a significant look should be directed at him, practise first in case it looks like leering).
I wouldn’t say you’ve had a senior moment as it’ll turn into you’ve got raging dementia in translation
J’avais la tête ailleurs?
Yes very good, implies just thinking about something else rather than senility.
Happens to me a lot!
Good suggestion, though aléatoire might be a better adjective in my case.
I often say, “Désolé, j’suis con!” 
That seems a bit beyond ironic self depreciation 
Also a bit early onset senility to me, so prefer the adverb. Much more temporary state of mind, rather than permanent dottiness
Ah, probably should have added a smiley, I wasn’t actually suggesting that one use the resultant phrase 
As I said, “J’avais la tête ailleurs” seemed perfect to my mind.