Short answer
Does an MLAT request have an age limit?
Not really. But there is little or no legal authority resolving this question definitively.
Long answer
Treaties on Mutual Legal Assistance in Criminal Matters (MLATs) enable
law enforcement authorities and prosecutors to obtain evidence,
information, and testimony abroad in a form admissible in the courts
of the Requesting State....MLATs require the Requested State to
provide the Requesting State with certain kinds of assistance or
evidence such as documents, records, and testimony, provided the
requirements of the treaty are satisfied. The process is streamlined
through the establishment of a “Central Authority” within each country
(Source)
Ultimately, what is covered by an MLAT depends upon the exact language of the treaty in question. An MLAT is typically a bilateral treaty between two countries only, so most countries have dozens of separate MLATs that apply to different countries. In order to write this answer, I reviewed a representative sample of MLATs, all of which tend to follow a similar model. This was possible because these treaties tend to be quite short.
Typically, a mutual legal assistance treaty (MLAT) is limited to "criminal matters", but expressly excludes military justice cases, national security cases, and political offenses.
Newer MLATs often also sometimes exclude petty offenses such as less serious misdemeanors and traffic offenses from their definition of "criminal matters".
On the other hand, MLATs often including prosecutions of violations of government regulations, and civil forfeiture cases, in their definition of "criminal matters". They do this either by including them in the express language of the treaty itself, or through statements in an official commentary to the treaty prepared during negotiations over the treaty between the countries that are parties to it.
The context of what is allowed in an MLAT in the treaty itself also makes clear that the term "criminal matters" includes suspected criminal matters where it has not yet been established definitively that a crime has been committed at all. Half of the reason for an MLAT is to assist law enforcement in one country in the process of investigating suspected crimes, just as they would domestically. An MLAT doesn't only pertain to prosecutions of already identified suspects whose crimes are already well established.
Apart from this very thin level of guidance, however, an MLAT typically does not tightly define what constitutes a "criminal matter".
Extradition treaties usually require that a person be extradited to a requesting country for a crime only if the producing country has an offense similar in character and severity of punishment to that of the requesting country. But MLATs typically do not require "dual criminality" to compel the producing country to assist the requesting country in obtaining evidence for a criminal prosecution by the requesting country.
Would the answers to the above also apply to the alleged victim, for
example they were too young at the time of the crime for an MLAT to be
used to investigate whatever crime they were a victim of?
The age of the alleged victim would be irrelevant to whether something was a criminal matter which an MLAT can be used to investigate or gather evidence regarding.
Nothing in the concept of an MLAT or the typical terms of an MLAT would suggest that this is the case. Nothing suggests that offense against juveniles, from child abuse to child molestation to theft from children, cannot be investigated using the MLAT process.
Would the alleged perpetrator's age be a factor, for example if they
were at the time of the crime (or still are) under the age of criminal
responsibility in either the UK or the other country. Could that
prevent an MLAT being used?
Would it be based on the age of criminal responsibility (nine years of
age in England & Wales), or the age that the country considers
citizens to be adults (I believe is 16 in England & Wales)?
There is little or no academic commentary, and little or no case law, addressing the question of whether juvenile delinquency offense that would be crimes if committed by adults count as "criminal matters" for purposes of an MLAT.
Ultimately this question would usually be resolved by the "Central Authority" in the state receiving a request for assistance under the MLAT interprets it.
The term "criminal matters" in an MLAT, however, is typically given a fairly broad definition to the extent that it is defined at all, as noted above.
Also, an MLAT is frequently used in criminal investigations before the identity or age of the offender or suspect is known.
For example, an MLAT could certainly be used to investigate a suspected bike theft gang that appeared to be operating across the border between the Netherlands and Belgium, even if there was a reasonable chance that some or all of the members of the gang were minors, but law enforcement wasn't yet sure who was involved in the gang.
So, the more plausible reading of the term "criminal matters" in an MLAT, in most cases, is that for purposes of the MLAT, an offense that would be a criminal matter if committed by an adult, does not cease to be a criminal matter merely because it was committed by a minor.
This would be a fair reading of these treaties, even if an offense may be official classified for domestic purposes as "juvenile delinquency" or something similar when it is committed by a minor, rather than being usually handled by the criminal justice system for adult offenders.
This being said, there certainly is a legitimate argument that could be made that juvenile offenses should not count as "criminal matters" for purposes of an MLAT, at least when domestic criminal laws make that distinction. MLATs are rarely clear on this point, and it is fair, to argue at least, that something which isn't defined formally as a "criminal matter" in the requesting country's domestic law shouldn't count as a "criminal matter" for purposes of the MLAT.
People suspected of engaging in conduct that would be a crime if committed by an adult, however, will not generally have standing to participate in the treaty implementation process. An MLAT is generally implemented through government to government interaction through each country's "Central Authority", in which a suspected criminal or investigation target has no involvement. And, as previously noted, decisions regarding what is covered by the treaty are usually going to be made by the Central Authority of the country receiving a request for legal cooperation from the Central Authority of another country.
So, juvenile's charged with offenses that would be crimes if committed by adults are in a poor position procedurally to argue that an MLAT should be interpreted in a narrow manner to exclude their case, and would have only a weak argument to support that position, even if they could.
