If one did not know the halacha that a minhag tova or saying to do a mitzvah is a neder automatically, and if he would have known he would have said “bli neder”.
Is that considered a neder btaus that does not even need hatarah?
What if a 11 year old girl or 12 year old boy did not know, does it require hatarah?
Also, can you please explain what falls under the guidelines of becoming a neder? Is it any good practice that you make for yourself or only something that is mentioned in the poskim?
One who performed a minhag tov 3 times is bound as a neder even if he was unaware of the halacha that states as such.
There is some doubt as to whether an 11 yr. old girl or 12 yr. old boy is bound by this halacha, see Shu”t Minchas Yitzchak 10:17 s”v veheenay who seems to take a stringent approach.
The question of what is included is also somewhat unclear, the Steipler Gaon in his letters Kreina Deigrisa 1:130 writes that one should be stringent with regards to any righteous practice.
It should be noted that the opinion of Rav Shlomo Zalman Auerbach was that the hatara nedarim of Erev Rosh Hashana may be relied upon for this form of neder.