Originally Answered: Could state statutes pertaining to juveniles be considered unconstitutional?
What you've asked actually is a quintessential constitutional law exam question regarding the Equal Protection Clause of the Fourteenth Amendment. Perhaps contrary to some expectations, the Clause doesn't forbid all laws that discriminate between two groups of people (e.g., children and adult), but rather only requires that they present a sufficient reason for making such a distinction in treatment.
The level of scrutiny to which we subject such laws and the need the government has to show to justify them depend on what categories of "discrimination" are drawn. For example, laws that treat people differently on the basis of race are viewed with the utmost skepticism by the courts and are very often struck down as unconstitutional. Certain other "suspect categories" are subject to heightened forms of scrutiny (e.g., gender, ethnicity, or national origin).
Other forms of distinctions merely require the government to provide a "rational basis" for its law. Age, wealth, political affiliation, and disability status are among these non-suspect classifications. So, a law treating people differently based on their age, such as the ones you've identified, is valid so long as the government can demonstrate that the distinction it draws is "rationally related to a legitimate government interest." In this case, permitting juvenile, but not adult, criminal records to be sealed is rationally related to the government's interest in affording children a special chance to change and rehabilitate themselves.
Certainly, one can argue that children aren't entitled to that unique treatment as a matter of policy, but the equal protection analysis doesn't ask that question. It merely asks whether the government's interest is legitimate, and the means used to effectuate it rationally related. One could dispute that determination, too, but the test is often so broad in practice that almost no law is found unconstitutional on rational basis review. For that reason, whatever your personal feelings, I think it very unlikely that a constitutional challenge to the sorts of laws you identify would be successful.