¿Cuántos años tienes? (16)
"How old are you?"
In English, we ask how old a person is. In Spanish, they ask how many years a person has. So, directly translated, this sentence is "How many years do you have?"
"Cuántos" is a question word used for counting and measuring, so it translates to "how many".
años = years
Tienes = (you) have. This comes from the infinitive verb tener (to have). "Tienes" is the informal "you" form of tener. So, asking someone "¿Tienes un perro?" is informally asking a friend "Do you have (tienes) a dog (un perro)?"