Why did the Jews comment that Jesus was not yet fifty years of age?
This is a very obscure subject to research accurately using Catholic sources. There is certainly no definitive statements on the subject whatsoever and as such Catholic interpretation on the subject matter is open to various points of view even within the Church.
That said let me do my best, which is a far cry from perfect or even well done, but here it is!
I can remember reading about this very subject some 25 years ago. I can not recall the exact texts or Catholic author, but the gist of what the Jews were getting at was the Our Lord has not yet attained the age of obtaining wisdom for he was still a young man. Being unable to find the exact references desired, I will do my best to explain it.
St. Ireaneus tries to explain what the texts means in his Adversus Haereses, book II, chapter 22, but admittingly it is somewhat confusing. “Being thirty years old when He came to be baptized, and then possessing the full age of a Master, He came to Jerusalem, so that He might be properly acknowledged by all as a Master.”
Part of the process of recapitulation is for Christ to go through every stage of human life, from infancy to old age, and simply by living it, sanctify it with his divinity. Although it is sometimes claimed that Irenaeus believed Christ did not die until he was older than is conventionally portrayed, the bishop of Lyon simply pointed out that because Jesus turned the permissible age for becoming a rabbi (30 years old and above), he recapitulated and sanctified the period between 30 and 50 years old, as per the Jewish custom of periodization on life, and so touches the beginning of old age when one becomes 50 years old.
In the passage of Adversus Haereses under consideration, Irenaeus is clear that after receiving baptism at the age of thirty, citing Luke 3:23, Gnostics then falsely assert that "He [Jesus] preached only one year reckoning from His baptism," and also, "On completing His thirtieth year He [Jesus] suffered, being in fact still a young man, and who had by no means attained to advanced age." Irenaeus argues against the Gnostics by using scripture to add several years after his baptism by referencing 3 distinctly separate visits to Jerusalem. The first is when Jesus makes wine out of water, he goes up to the Paschal feast-day, after which he withdraws and is found in Samaria. The second is when Jesus goes up to Jerusalem for Passover and cures the paralytic, after which he withdraws over the sea of Tiberias. The third mention is when he travels to Jerusalem, eats the Passover, and suffers on the following day.
Irenaeus quotes scripture, which we reference as John 8:57, to suggest that Jesus ministers while in his 40s. In this passage, Jesus' opponents want to argue that Jesus has not seen Abraham, because Jesus is too young. Jesus' opponents argue that Jesus is not yet 50 years old. Irenaeus argues that if Jesus was in his thirties, his opponents would've argued that He's not yet 40 years, since that would make Him even younger. Irenaeus' argument is that they would not weaken their own argument by adding years to Jesus' age. Irenaeus also writes that "The Elders witness to this, who in Asia conferred with John the Lord's disciple, to the effect that John had delivered these things unto them: for he abode with them until the times of Trajan. And some of them saw not only John, but others also of the Apostles, and had this same account from them, and witness to the aforesaid relation."
In Demonstration (74) Irenaeus notes "For Pontius Pilate was governor of Judæa, and he had at that time resentful enmity against Herod the king of the Jews. But then, when Christ was brought to him bound, Pilate sent Him to Herod, giving command to enquire of him, that he might know of a certainty what he should desire concerning Him; making Christ a convenient occasion of reconciliation with the king." Pilate was the prefect of the Roman province of Judaea from AD 26–36. He served under Emperor Tiberius Claudius Nero. Herod Antipas was tetrarch of Galilee and Perea, a client state of the Roman Empire. He ruled from 4 BC to 39 AD. In refuting Gnostic claims that Jesus preached for only one year after his baptism, Irenaeus used the "recapitulation" approach to demonstrate that by living beyond the age of thirty Christ sanctified even old age. - Irenaeus (Wikipedia)
They were certainly not discussing his age with any precision accordingly. They simply gave good measure. Jesus was still a young man. He could not claim even to be one of the elders. How then could He possibly have seen Abraham? Notice that the Jews do not repeat Jesus exactly. He speaks of Abraham seeing His day, they of His seeing Abraham.
But the phrase “fifty years” was the period of full manhood in the ancient Hebrew way of thinking (Numbers 4:3; Numbers 4:39; Numbers 8:24). This could expressed in such numbers as 50 years old, and there is no care to be more exact in comparison with the two thousand years which had passed since the close of Abraham’s earthly life. The thought of those present could be thus seen as: “Thou art still a young man, and hast thou seen Abraham who died twenty centuries ago?”
We can see that in Numbers 8:25 the Scriptures speak of the retirement of the Levites and as such are honoured for their past service.
Retirement for Levites
23And the Lord spoke to Moses, saying: 24This is the law of the Levites: From twenty-five years old and upwards, they shall go in to minister in the tabernacle of the covenant. 25And when they shall have accomplished the fiftieth year of their age, they shall cease to serve: 26And they shall be the ministers of their brethren in the tabernacle of the covenant, to keep the things that are committed to their care, but not to do the works. Thus shalt thou order the Levites touching their charge.
Jewish tradition suggests that at the age of 50, one would be considered to be old, wise and an elder.
At forty, one achieves (bina) understanding, at fifty, one is prepared to give (aitzah) wise counsel, at sixty, one is given the deference of seniority (zikna), at seventy, one is considered a sage (sayva).
And what of 50 and beyond? What is aitzah, zikna and sayva? It means becoming an elder who earns eternity by teaching, by mentoring, by leading. - THE WISDOM OF JEWISH ADULTHOOD
Now for the Gematria of Nun.
Gematria of Nun
According to Jewish gematria, Nun represents the number 50 - a number representing freedom and fullness of life.
50 days from the Exodus to the giving of the Torah
(50 days for the count of the Omer)
50 years for a Jubilee Year (yovel)
50 references to Exodus in the Torah
50 years of age before one has wisdom
This last point is where I am actually going. (Although the actual source evades me at the moment, I will try to find the reference desired, but it will time quite some time.) What the Jews were possibly getting at is that Our Lord was not truly considered an elder, for he was too young to have obtained true wisdom.
ELDER (Heb. זָקֵן, zaken). In Israel, as among all other ancient peoples, the elder is not only a person of advanced age, but also a man of distinct social grade (cf. šībum in Akkadian, senator in Latin, geron in Greek, and sheikh in Arabic). The elders were the consulting body of the city, the nation, or the king respectively, and as such were considered "the wise" (cf. Ezek. 7:26 with Jer. 18:18). As a social institution, various types of elders are named: elders of a people (Israel, Judah, Moab, and Midian, Num. 22:4, 7; Egypt, Gen. 50:7); elders of an area (Gilead, Judg. 11:5–11); elders of a tribe (Deut. 31:28); elders of the Diaspora (Jer. 29:1); elders of the priests (II Kings 19:2; Jer. 19:1); elders of the city (passim); and elders of the house (i.e., palace, Gen. 50:7; II Sam. 12:17). The most prominent are the elders of the people or the country and the elders of the city. - Jewish Virtual Library