Abu Dawud

Benefits and Issues:
From the aforementioned hadith, it is clear that the Prophet (sallallahu alayhi wa sallam) commanded the ummah to slaughter a musinnah (two-toothed) animal as a sacrifice.
And in times of difficulty and hardship, he granted concession to sacrifice a jadha’ (young animal), but from other narrations it is understood
that even in normal circumstances, when obtaining a musinnah (two-toothed) animal is not difficult or hard, a jadha’ can be sacrificed.
As Hazrat Uqbah bin Amir (radi Allahu anhu) narrates that we sacrificed a jadha’ sheep with the Messenger of Allah (sallallahu alayhi wa sallam).
(Sunan an-Nasa’i, al-Dahaya, Chapter: al-Musinnah wa al-Jadha’, Hadith: 4387) And in Sunan Abi Dawud, Asim bin Kulayb narrates from his father
that we were with a companion of the Prophet (sallallahu alayhi wa sallam) whose name was Mujashi’. When goats were distributed for sacrifice, they became insufficient.
So he ordered a caller to announce
that the Messenger of Allah (sallallahu alayhi wa sallam) used to say:
Indeed, a jadha’ (one-year-old) suffices in place of a musinnah (two-toothed).
(Sunan Abi Dawud, al-Dahaya, Chapter: What is Permissible in the Age of Sacrificial Animals, Hadith: 2799) And similarly, Umm Bilal (radi Allahu anha) narrates
that the Messenger of Allah (sallallahu alayhi wa sallam) said: Sacrifice the jadha’ of a sheep,
for its sacrifice is permissible.
(Musnad Ahmad: 368/6) From the above-mentioned ahadith, it is understood that even in normal circumstances, the jadha’ of a sheep can be sacrificed; however, according to the narration of Hazrat Jabir (radi Allahu anhu), sacrificing a musinnah (two-toothed) animal is superior.
As Hafiz Ibn Hajar states regarding this in Fath al-Bari:
Imam Nawawi has transmitted from the majority of scholars that they have interpreted this hadith as indicating superiority.
(Fath al-Bari: 20/10) (Jadha’) This is only permissible in sheep (ram, wether).
It is not permissible to sacrifice the young of other animals at this age.
The Messenger of Allah (sallallahu alayhi wa sallam) granted concession and permission to a few companions (radi Allahu anhum ajma’in) in cases of necessity, and along with that he said:
After you, it is not permissible for anyone else to do so.
(Sahih al-Bukhari, al-Adahi, Hadith: 5556) And it is also possible that in the beginning, both types of jadha’ were permissible, but later the sacrifice of the jadha’ of a goat was prohibited.
The jadha’ of a sheep (ram, wether) can be sacrificed,
as is clear from the above evidences.
But there is a difference of opinion regarding its age.
Some have stated the period as one year,
some six months, and some seven months.
Imam Nawawi states regarding this:
The most preferred opinion regarding the age of a jadha’ is that its age should be a complete one year.
(Kitab al-Majmu’: 365/8) Hafiz Ibn Hajar states regarding this:
According to the opinion of the majority, the jadha’ of a sheep (ram, wether) is one whose age has completed one year.
(Fath al-Bari: 21/10) Therefore, those who wish to sacrifice a sheep (ram, wether)
should definitely keep in mind
that its age should be at least one year.

(1) It is recommended (mustahabb) to sacrifice a two-toothed animal (muthanna). In the case where a musinnah is not available or one does not have the means, it is also permissible to sacrifice a jadha‘ (young sheep). There are differing opinions among the scholars regarding the age at which a jadha‘ becomes suitable for sacrifice. The majority of scholars and hadith experts hold the view that its age should be one year or close to it. It is also understood that the jadha‘—that is, a fully weaned young sheep—can only be sacrificed from among sheep, not from goats, cows, or camels. The wording of the hadith [فتذبحُوا جذعةً من الضأنِ ] is a clear and solid proof of this. This is also the opinion of the scholars, hadith experts, and others.

(2) An animal whose teeth have fallen out is called musinnah or thani in the Arabic language. In Urdu, it is called “do danta” (two-toothed), and in Punjabi, “donda.” Some individuals have interpreted musinnah to mean “one year old,” but this meaning is neither correct linguistically nor according to common usage, because the word musinnah is derived from “sin,” which means “tooth,” not from “san,” which means “year.” In common usage as well, a goat does not become two-toothed at one year; it usually happens later. Rarely, it may happen at one year, but generally, it does not. The ruling is based on what is generally the case. The main objective is the falling of the tooth, not the age, because there is no fixed age for the teeth to fall out, and determining the age is also difficult. There can be disagreement in this, and a person selling the animal may even lie about the age, but the falling of the tooth and the emergence of a new tooth is a clear and definite sign in which fraud is not possible. Therefore, the correct view is that the sacrificial animal should be two-toothed (donda), whether it is a goat, cow, or camel, and all these animals become two-toothed at different ages. However, if such an animal is not available or one does not have the means, then the sacrifice of a jadha‘ sheep is also permitted, but it is necessary that it be fat and close to being two-toothed. Some people have tried to specify the age, with opinions ranging from six months to one year. To avoid doubt and uncertainty, one should not sacrifice a sheep or ram less than one year old. In the lexicon, the opinion of one year is more well-known, and the majority of scholars have adopted this. Rationally as well, this is correct, because in the absence of a two-toothed animal, the effort should be to sacrifice an animal as close as possible to it, not one of six months, which is much less than two-toothed.

Benefits and Issues:
Shaykh al-Albani rahimahullah explains that in the hadith of Hazrat Mujashi‘ radi Allahu anhu, the term "jadh‘a" refers to a jadh‘a of a sheep, not a jadh‘a of a goat.

When Hazrat Abu Burdah radi Allahu anhu slaughtered the sacrificial animal before the Eid prayer, the Messenger of Allah sallallahu alayhi wa sallam said:
"This is a goat for meat (not for sacrifice)."
He (Abu Burdah) said:
"O Messenger of Allah sallallahu alayhi wa sallam! I have a jadh‘a of a goat (can I offer it as sacrifice?)"
The Messenger of Allah sallallahu alayhi wa sallam said:
"Sacrifice it, but it will not be valid for anyone after you." (Sahih al-Bukhari, al-Adhahi, Chapter: The statement of the Prophet sallallahu alayhi wa sallam to Abu Burdah: 'Sacrifice the jadh‘a from the goats, it will not suffice for anyone after you.' Hadith: 5556)

Shaykh al-Albani has derived from this that, in light of the hadith of Hazrat Abu Burdah radi Allahu anhu, the jadh‘a of a sheep (a one-year-old lamb whose teeth have not fallen out) is permissible.
And this permissibility is not conditional upon the absence of a two-toothed (musinnah) animal, rather it is absolutely permissible.
And Allah knows best. See: (Hashiyah Da‘if Sunan Ibn Majah, the hadith under discussion, as well as the benefit of hadith: 3154)