This sign can also be understood by those who don't speak English. It shows two elderly people. As we now know, red stands for Careful. And there's the answer. Below the sign it says: "Elderly people" Because their eyes are sometimes not so good and they move more slowly, there is a special sign for them in Great Britain. You often see it when approaching retirement homes, and it tells you to be extra careful.
You've probably never seen this sign for real. We're fairly sure of this, because it stands by an Irish lake named Acres. It's the site of the first floating boardwalk. If you pass this sign, you are entering more than 200 kilometres of waterways and lake shores. So it was put there to make sure that nobody accidentally ends up with wet feet.