Of course, I was shocked, as I had been used to not asking about each other's age, except among best friends.
To make it not too awkward, I joked, "Too old to tell you." At least he was not mad that I did not answer directly.
Later, some other friends told me that I could have said, "Take a guess." as well.
That was in the Philippines, and I guess he was about 18-19 years old, more or less a new high-school graduate.