In short, without direct physical evidence (forensic or medical examination), testimony, or documentation to act as proof, you are left up to the discretion of the court to determine whether evidence is admissible and up to a jury to decide if it is reasonable to presume that a person depicted is of a certain age if no other evidence to the contrary is provided. This is beyond the objective nature of science.
Please be aware, that as you are making this question specifically about criminal evidence, I am addressing that aspect of your question. There are ways of dealing with and presenting evidence that may preclude the need for any scientific proof of age, as it is up to the determination of reasonableness by the jury that is the overriding factor. If the jury does not reasonably feel that the person depicted is a minor as may be the case if the actual identity of the victim cannot be established, or if the accused could not have reasonably known that the person was a minor, unable of providing legal consent, then they can vote to acquit the accused.
Video’s Role in the Criminal Justice Process: DRC to ICC
Since documentary and forensic evidence could not prove age, the Prosecutor relied, in part, on a series of video clips showing: i) Lubanga at the military training camps speaking to children; and ii) children serving as his bodyguards. A sample of the footage from the opening argument can be watched here and below.
You will also see if you follow this link that the normal clinical determinant of age, examination of growth plates is not relevant as that data was generated on American and European populations and is likely different for African children, as their access to nutrition and the childhood illnesses that they face likely have much more to do with the changes in bone plates that can be used to determine age forensically.
While this example is related to an international court of law hearing a war crimes tribunal, the charge given to most juries would be that they are the judges of the facts and if a reasonable person would arrive at a certain conclusion.
From the Connecticut Supreme Court Jury Instructions. Most US States will have similar statutes and guidelines.
You will decide what the facts are from the evidence that will be presented in this courtroom. That evidence will consist of the testimony of witnesses, documents and other material admitted into evidence as exhibits, and any facts on which the lawyers agree or that I may instruct you to accept.
The following are not evidence and you must not consider them as evidence in deciding the facts of this case:
statements and arguments by the attorneys,
questions and objections of the attorneys, and
testimony that I instruct you to disregard.
There are two kinds of evidence: direct and circumstantial. Direct evidence is testimony by a witness about what that witness personally saw or heard or did. Circumstantial evidence is indirect evidence; that is, it is evidence from which you can infer another fact. As an example: if you wake up in the morning and see that the sidewalk is wet, you may infer that it rained during the night. The wet sidewalk is circumstantial evidence that it rained. Other evidence, however, may provide another explanation for the water on the sidewalk, such as a garden hose that was left on overnight. Therefore, before you decide that a fact has been proved by circumstantial evidence, you must consider all the evidence in the light of reason, experience and common sense.
In deciding this case, you may consider both direct and circumstantial evidence. The law permits you to give equal weight to both, but it is for you to decide how much weight to give to any evidence.
Some evidence may be admitted for a limited purpose only. When I instruct you that a piece of evidence has been admitted for a limited purpose, you must consider it only for that purpose and for no other.
2.2-3 Reasonable Doubt
The meaning of reasonable doubt can be arrived at by emphasizing the word reasonable. It is not a surmise, a guess or mere conjecture.1 It is not a doubt raised by anyone simply for the sake of raising a doubt. It is such a doubt as, in serious affairs that concern you, you would heed; that is, such a doubt as would cause reasonable men and women to hesitate to act upon it in matters of importance.2 It is not hesitation springing from any feelings of pity or sympathy for the accused or any other person who might be affected by your decision. It is, in other words, a real doubt, an honest doubt, a doubt that has its foundation in the evidence or lack of evidence.3 It is doubt that is honestly entertained and is reasonable in light of the evidence after a fair comparison and careful examination of the entire evidence.4
EDIT: Due to a moderator edit of the question, the following section is no longer valid to the question and has been stricken
As you can see from this statute 7.4-3 Obscenity as to Minors -- § 53a-196 it is up to the discretion of the jury to determine whether or not the person depicted in the video was a minor and that the person accused of the crime knew that the person was a minor.
In summary, the state must prove beyond a reasonable doubt that 1) the (material / performance) was obscene as to minors, 2) the defendant promoted the (materials / performance) to another person, knowing of its character and contents, and 3) knew that the other person was a minor.
If you unanimously find that the state has proved beyond a reasonable doubt each of the elements of the crime of obscenity as to minors, then you shall find the defendant guilty. On the other hand, if you unanimously find that the state has failed to prove beyond a reasonable doubt any of the elements, you shall then find the defendant not guilty.
As a result, even if the prosecutor can prove that the person depicted is a minor, it may still rely on whether or not the state can prove that the accused knew the age of the person they are accused of exploiting.
Part 7 subsection 7 is related to how the State of Connecticut instructs juries related to Child Pornography
So, from a legal perspective, it is highly subjective, and even from a clinical perspective, it is likely that there are different metrics related to different populations and how children's developmental conditions affects their bone structure, onset of puberty, and display of secondary sexual characteristics.