“What if one has no mesorah or minhag avos?”
Exactly. Plus, nowadays there is a lot of “inter-marriage” (chassidish/litvish litvish/sfardi) so minhagim get combined.
Also, boys go away to Yeshiva at a young age and learn from Rabbeim and take on the Minhagim of the Yeshiva they belong to. There is nothing wrong with this.