I find it hard to completely trust a conscienceless government to keep appropriate mistake-free records. All people as a whole must take some personal ownership and responsibility in a governmental system for the system to work the best. The more people there are that look at government as a handout and/or security measure, the more the people lose freedoms.
Laws differ from state to state, country to country, and year to year.
For example, a Senior in High School falls for a Sophmore... they make mistakes... the senior graduates, ages over 18, but they continue to be a couple. The parents of the younger find out about the serious relationship, and decide to press charges for statutory rape. Older person in this case is in many states permanently tagged with "Sex Offender" status, and force to register in every place he lives for the remainder of his days. The two participants still get married after the younger one is an adult. They have children. They both struggle for years unsuccessfully to get the label removed.
In another case, another 18 year old falls for a 14 year old, but these parents give consent at 16 for the two to be wed. No charges. No label. They have 3 babies by the time the younger is 20 and the elder is 24.
I have friends that marry as far as 12 years apart. My own parents are 6 years apart. In some contries, arrangements are still legal much younger than this.
What about a case where a younger person intentionally mis-represents age, complete with fake ID, and actively pursues a life-style or actions that end up getting an adult in trouble.
Yes, there are examples of circumstances where I'd like to know: we had a convicted male pedophile nearby trying to hire Boy Scouts to clean his house and yard. We took immediate action and got his parole officer involved. I have no idea what he was actually convicted of, only that it worried us enough that we felt we needed to guard against it.
I have seen all of these examples happen.
I am not against a registry, or the collection of data. However, the point of the problem is, no matter where you draw a merciless line, people will be affected. And currently, I feel the ability of any public body to forgive and forget is swung very far in the merciless direction, to include more and more aspects of our digital lives. Online ad-trackers and ID theives probably know more about individuals than any governmental body. There really are no secrets. But there comes a time when the eyes of necessity should be half-closed.