What is the ruling on janabah if doubts arise about having performed ghusl from it after reaching puberty?

Do not pay attention to doubts and thoughts regarding the validity of an act of worship after completing it, for the default is that it is valid. Jurists have explicitly stated that doubt concerning an act of worship or one of its conditions after its completion does not affect it, in order to avoid hardship, because the apparent reality is that it was performed correctly.