The gemara says on 18a towards the end of the amud that the gemara is speaking about when a katan made a claim and I think the ritva in gitin speaks out that the reason why the gemara calls it Ta'anas Atzmo over here is because when a katan comes against you with a Ta'anah you don't usually have to answer him back and from the fact that you did it's called Ta'anas Atzmo and the gemara continues and says that the whole argument is in rabah and the gemara says on 18b that reb eliezer holds it doesn't make a difference between he himself or his son they both won't be Mei'iz Panav and rashi says that the reason why the gemara calls it Ta'anas Atzmo is because normally a katan doesn't have a valid Ta'anah.
My question is that the gemara here is saying that in this case the Katan does have a valid Ta'anah and you have to make a Shevuah when he comes along and Ta'anah's against you so I don't understand why rashi says that the reason why it's called Ta'anas Atzmo because normally a katan can't make a Ta'anah but over here he HAS GOT A VALID Ta'anah so it's not Ta'anas Atzmo it's Ta'anas Acherim?
Benzi , London
1) Rashi agrees that in this case the minor has a valid claim, but since in most cases a Katan cannot make a Ta'anah, the word used is "Ta'anas Atzmo." It is just a question of etymology. Since the majority of arguments of Ketanim are invalid, the language used to describe their Ta'anos follows the majority, even in the minority case where the Ta'anah is valid.
2) Rashi himself in Gitin (51b, DH Al) writes something similar to what you wrote in the name of the Ritva in Gitin. Rashi writes that at the beginning of the Sugya, the term "Ta'anas Atzmo" implies that no one is claiming anything from him.
3) Rashi in Shevuos (beginning of 42b, DH Ela) explains a little more about what he meant with what he wrote here in Kesuvos. Rashi there writes that it is termed "Ta'anos Atzmo" here because in other places the Ta'anah of a minor is not a valid argument, even though in this case it is a valid argument.
4) We can understand better what Rashi is saying with the help of the principle, "Chazal Al ha'Rov Yedabeiru" -- when Chazal said something it does not cover every case, but it covers the majority of cases. An example of this may be found in the Minchas Chinuch, Mitzvah 530:22, where he discusses the Mitzvah of Eglah Arufah when a corpse is found in the field. What is the Din if one finds a one-day-old baby who has been murdered? According to the Gemara in Sotah 46b (that the Sanhedrin must say, "We did not let this person go on his journey without providing him with food"), it would appear that Eglah Arufah does not apply to a newborn baby because one clearly cannot provide him with food. However, the Minchas Chinuch writes that this is not the law. For every corpse found, an Eglah Arufah must be brought. Even though the Beis Din says that they provided him with food, this does not bother us, because "Chazal Al ha'Rov Yedabeiru" -- when Chazal required Beis Din to say that they provided him with food, this is referring to the majority of cases, but it is possible that there are exceptions to the rule.
5) Similarly, the term "Ta'anas Atzmo" is used by Chazal because in the majority of cases of a claim of a minor it is an accurate description. Even though in a minority of cases it is not an accurate description, that does not bother us because "Chazal Al ha'Rov Yedabeiru."
(In fact, Rashi here in Kesuvos and Rashi in Shevuos are good sources, bs'd, for the rule of the Minchas Chinuch.)
Kol Tuv,
Dovid Bloom
So why does the gemara here give an example of Ta'anas Atzmo if it doesn't apply over here?
The phrase "Ta'anas Atzmo" is always used to refer to the claim of a minor, even in a case where one is required to answer such a claim. Despite the fact that one must answer the claim -- which therefore means that it does not fit with the usual meaning of Ta'anas Atzmo, which generally means an argument which is not necessary to answer -- nevertheless Chazal followed the majority of scenarios when using their language, and since one does not usually have to answer the Ta'anah of a Katan, it is always termed a "Ta'anas Atzmo" even in a case where one is obliged to answer the Ta'anah.
The best example I can think of to illustrate the above idea is from the Israeli Yeshiva world: There are a lot of non-Israelis studying in Yeshivos in Eretz Yisrael, and the average Israeli will often refer to them all as Americans. Even though there are plenty of different nationalities here, the tendency is to describe all of the foreigners as though they are all the same, and therefore a lot of Israelis call all foreigners "Americans" even though in reality they originate from all different countries.
This is how "Ta'anas Atzmo" is used. In the majority of occasions a minor makes a Ta'anas Atzmo which one does not have to answer for. Even on occasions when one is obliged to answer the Ta'anah of a Katan, it still retains the same name, "Ta'anas Atzmo," because the majority usage of the word overrides the minority usage. Since most Ta'anos of a Katan do not require an answer, and are therefore a Ta'anas Atzmo, we still retain the same phrasing even when one is required to answer the claim.
Kol Tuv,
Dovid Bloom