Most offenses have an attached statute of limitations (French: prescription) which limits how much later after the act one can be sued for past actions.
In France, this can range from a year (for "contraventions", e.g. minor offenses) to unlimited (for crimes against humanity), with most cases at 6 years for misdemeanours (délits) or 20 years for crimes.
For some misdemeanours against children, the statute of limitations is extended to 10 or 20 years, and only starts at the child's majority.
Also, the statute of limitations has changed many times over the the last 30 years, and depending on the date of the offense, the type of offense, who committed it (a parent, a teacher, etc.), the associated statute of limitations may or may not have been extended. The exact rules are complex, there's a calculator here. Based on the info you gave, it would seem that in your case the statue of limitation ends when you turn either 38 or 48, depending on the details.
Note for instance that an "agression sexuelle" (a misdemeanour) will not have the same statute of limitations as a "viol" (a crime). Part of the procedure will be to evaluate whether the offense was one or the other, as that has a big impact on how it is handled (statute of limitations, type of trial...).
There are several ways you can start the process:
- You can just go to the police (or gendarmerie) and file a complaint
- You can file a complaint with the "procureur de la république", usually with "constitution de partie civile"
- You can contact a lawyer first, who will advise you on the best route forward. Under certains conditions you can ask for legal aid (French: Aide juridictionnelle) to pay for the lawyer.
The fact that multiple countries are involved does complicate things, as there may be territoriality issues based on where the offenses happened, and proceedings against people in different countries are often more complex (it may make sense to file the complaint in the country where the offender lives, otherwise France may need to request extradition of the offender, which is a process which can take ages).
I strongly recommend you talk to a lawyer first, or contact one of helplines which specialise in these cases. I'm not sure which ones are the best suited, though, and that may depend on your specific situation. This may make the difference between your complaint being dismissed (classée sans suite) for some reason or other and an actual trial, damages, etc.