Is ghusl required if it is highly probable that the traces of احتلام (wet dream) are old?

Ghusl al-janabah (ritual bath after sexual impurity) is not obligatory unless its cause is definitively established. It is not necessitated by mere doubt. So, if one has a wet dream but does not see semen, or doubts whether semen was discharged, then ghusl is not required of them. Therefore, your prayer is valid, as the preponderance of conjecture has a role in acts of worship.