“You are not yet fifty years old,” the Jews said to him, “and you have seen Abraham!”
“fifty years old” = Fifty was the age at which the Levites retired (Num 4:3), and the age at which a man was thought fit to give counsel, the prime of life. It was just a good measure to indicate that Jesus wasn’t even old enough to have fulfilled duties as a Levite, much less be an elder, and even more greatly removed is the possibility of His being around when Abraham was alive 2000 years previous. Jesus was still a young man; He had begun His ministry when He was about 30 years old (Luke 3:23) and His ministry only lasted about 3 1/2 years.
Numbers 4:3 Count all the men from thirty to fifty years of age who come to serve in the work in the Tent of Meeting.
Luke 3:23 Now Jesus himself was about thirty years old when he began his ministry. He was the son, so it was thought, of Joseph, the son of Heli,
“seen Abraham” = Jesus had spoken of Abraham seeing His day; they misunderstand and take it as His seeing Abraham’s day.