Is 22-23 too young to be an e6?

First, I would not consider a SSG an upper leader. This is still a junior NCO. SFC and up are Senior NCOs.

That being said, the age, to me, is not the important thing. What is that NCO's experience level? Maturity? Wisdom?
Generally speaking, when we think of age as it relates to leadership, we think of older being better. This is because (again, generally speaking) with age comes maturity and wisdom. But... I have met 22 year-olds who were wiser and more mature than most 55 year-olds and 40 year-olds who acted 14.
If that 22 or 23 year old has the skills, knowledge, and maturity to do the job... let them.

Rank without experience is like a racecar with no windshield. Sure it goes fast, and it might seem impressive but it's usually without direction and quite possibly flying into a wall with an accompanying Michael Bay'esque fireball..

It is not about age. It is more to do with displaying ones abilities in their technical craft , while exhibiting the ability to lead, train and care for Soldiers. Age does not directly translate to maturity, nor is it a marker that is considered in promotion.

This is a much deeper topic and I could be on all philosophical, the simplicity encapsulates your answer.

No.

Seems to me, if you see someone in Special Forces or Ranger units, they tend to get promoted faster than others. I have a good friend that was a SGT Major at 37ish. He was with SF the majority of his career. So, no, it isn't impossible. Also, if the soldier isn't in SF, but got a Bachelor Degree during the first few years of his enlistment, that ways heavily on Promotion points.

I was a 20 year old Sgt when i was in the Marines in 1980.

Depends on the person, not the age. I have met childish 40 year olds, and 20 year olds who had the wisdom of seasoned veterans. Auddie Murphy was promoted to SSG at the age of 19 or 20 (he made SSG in 1944. keep in mind he falsified his age to join the Army in 1942 at or about 17.

This isn’t an age question, it’s a maturity question. Completely depends on the individual. I know plenty of 40 year old SSGs/SFCs that act and live well below their ranks and age.

While many still use chronological age as one of the defining measurements of maturity and ability I have found that individual actions serve as a better indicator of who is ready for more responsibility and advancement. I was pinned a SSG down in ranger school, for about two minutes and I was only 22. Had been a rifle team leader as a SP4 as well as a squad leader at that same rank. I also served as a PSG while a SSG for a few years. My actual Maturity has been in question to this very day :)

I was an E-6 at 21 in Vietnam, I was not the youngest in my unit