Remember, this isn't intended to validate my age, it's intended to test my honesty and make sure that I'm not an autobot. Another way they use is having "don't know" as a possible answer to a question where it's not (or shouldn't be) a reasonable response. As an example, you might be given a drop-down list of professions, where the last two are "none of the above" and "don't know." The first one is quite plausible, especially if you're retired, as I am, but how likely is it that you're not going to know what your profession is? Basically, any time I see the same question in two different forms on different pages I presume it's an honesty check.
And, your method of always adding exactly ten years to your age will work, but only if you remember to do it each time, and how many years you added. Not all minors are going to realize that there might be a second age test, making this both an age and an IQ test.