I'm getting to the age at 55.
I can't remember having flu. I've had colds and recently had one that wasn't bad, but it stuck around for a couple of months in the background and kept coming back and needed a bit of anti-biotics to help shift it in the end.
I'm not sure how clear it is whether you have flu or one of the other myriad of respiratory viruses that circulate.
Bit of a snotty nose - a cold. Add in a temperature, aches and pains and flu tends to be self-diagnosed.
Late 1986 / early 1987 my wife had what we believe to be flu. She's still has bad memories of it. The usual symptoms, but in extremis. I had to help her to the toilet for a couple of days at one point and stay in attendance in case she keeled over.

I suppose when I start getting the letter I'll start thinking about having the annual shot as it isn't nice if you get it.