So this variable has equal intervals. Philosophers or astronomers may have answers for these questions, but in practical terms, there is no zero point. Hence, time in years would be an interval-level variable. The highest level of measurement is the ratio level. Even though the sample you are examining may not include any cases in the category zero, zero is possible at least in theory.
Age is another example.
After the first three criteria are met, we then determine if the variable has a zero point. If so, we consider the variable to be ratio-level. The zero point means we can sensibly multiply and divide the categories of a ratio-level variable. A person who has one child has half as many children as those with two children. These statements make sense.