He distinguished himself as a youth at Shittim with his zeal against the Heresy of Peor.
This line is in multiple places, including Wikipedia and The Israel Bible. It implies that Phinehas (aka Pinchas) is under 20 years old (since that's the age used for adult men elsewhere) when this event took place. The Hebrews were in Shittim at the end of the Exodus journey, which means Phinehas would have been born about halfway through the 40 years. Even if you count men in their 20's as youths, Phinehas would have to be born about a decade after the Hebrews left Egypt.
But then we have:
Eleazar, the son of Aaron, took himself [one] of the daughters of Putiel to himself as a wife, and she bore him Phinehas. Exodus 6:25
This line appears before the Hebrews leave Egypt. Does it mean Eleazer married and had one child before the Exodus? Or is this an out-of-order genealogy told earlier for organizational purposes? (We already know, and Rashi cites it repeatedly, that the Torah does not report everything in exact chronological order.)
In addition to the idea that a man in his 40's is really not a youth, we have the issue of Phinehas's mother.
[one] of the daughters of Putiel-: Of the seed of Jethro, who fattened (פִּטֵּ ם) calves for idolatry (see Rashi on Exod. 2:16) and [who was also] of the seed of Joseph, who defied and fought (פִּטְפֵּט) against his passion [when he was tempted by Potiphar’s wife]. — [from B.B. 109b] - Rashi's commentary on Exodus 6:25.
Rashi and many other commentators state that Eleazar's unnamed wife is one of Zipporah's sisters, another daughter of Jetro (Moses' father-in-law).
But here's the thing...when would Aaron's son have met his future wife?
Moses left Egypt when he was 40 years old. He stayed in Midian for 40 years. He had little to no contact with his family back in Egypt. I suppose it's possible there were messages prior to when he prepared to leave, but certainly no meetings or serious conversations.
When Moses decided to return to Egypt, he took his wife and two sons with him. But he didn't make it very far with them. They went back home before Moses continued on alone to meet with Aaron. Moses then returned with Aaron to the Nile Delta and stayed for perhaps as long as a year.
After the Hebrews made it to Mount Sinai, Jetro went to visit them, bringing Zipporah and the two boys.
This would have been the earliest time that Jetro could have brought another one of his daughters with him to marry off to his son-in-law's family (in this case, Moses' nephew). Moses might have told Aaron and Elisheba about his sisters-in-law and encouraged a match, but it couldn't have actually happened until Mount Sinai.
So, do we assume that the Torah is being strict with the chronography and somehow Jetro sent his daughter to marry a slave in Egypt before Moses even returned there?
Or can we assume that the Torah has given the genealogy early because it is important and it made organizational sense to include it there, but that this marriage and childbirth actually happened later?
[Note: we have a possible duplicate with Was Pinchas born in Egypt? but it does not talk about the issue of Phinehas' mother or give the level of answer I'm looking for.]