Student asks for letter of recommendation: I can't remember the student

I would try to accommodate the student. Most faculty were traditional college students, so this might seem like a weird request from our point of view, but many students don't have the same backgrounds, expectations, and economic resources that we do. Some students may need to take a long break from school due to economic setbacks, or because they've had a kid, or because of health problems. Also, keep in mind that not all grad schools are elite PhD programs, so the function of a recommendation letter may not be to vouch for the student's brilliance and ability to do groundbreaking research. For many programs, the purpose of the letter may be more to make sure that the student isn't a cheater or a psycho.

Be up front with the student. Explain honestly that you don't remember him. Let him know that if he has other possible recommenders who would have had more recent interactions with him, he would be better off with them. But if that's not possible, ask him for more detailed written info about his life and plans, and write the best letter you can. You may want to show the student the letter before you send it out, so he has an idea of what he's getting.

Assuming that the student had good grades, I'd explain that I couldn't remember the student and that I could only base my recommendation on the transcript. I'd still be willing to write such a letter, but I'd encourage the student to find someone else to write a letter if possible.

If the student had grades that weren't very good, then I'd simply decline to write a letter.