The Myth That All Teens Have Insanely High Testosterone
OP summarizes it as:
(There's a) myth that “you will always make the BEST gains when you’re in your teens and early 20s because that’s when you have sky-high test levels”. This is simply not always the case. Take me, for example. As a natural I have been struggling all my life with terrible acne, developed gynecomastia, and as of this last year was feeling very tired and lazy all the time. I have never touched a recreational drug of any kind in my life... I talked to my doctor, got a blood test, and it came back with a total testosterone level of 326.
I've long suspected this is a myth as well, and in a past post state that aside from potentially limiting growth velocity, I can't find any data on PEDs harming them young'uns (specifically testosterone, and above and beyond the normal risks). It's not a criteria I would use for judging readiness for a cycle, either. After all, hygonadism in children is often treated with testosterone without major issues.
The bro-science of waiting until you're 25 probably originates from the logical, and wholly accurate, idea that you should be mature enough to make a decision that is life altering. For many this tends to happen in their mid-20s. Over time, this tenet slowly morphed into using PEDs when under a certain age (usually 25) is fundamentally unhealthy and will do long-term damage.
Actual Testosterone Numbers By Age
So what is normal based on age? A meta study looked at this in men from ages 3 - 90. Turns out that age related decline isn't as obvious when we graph it out, and that average testosterone levels peak at 15nmol/L at around 20 years of age. Test levels definitely shoot up from 12 - 16 years old, but again aren't crazy high.
A cleaner way of looking at it is by percentiles, which shows the kinds of variation we see in males at all ages. There's a ~20% greater amount of Test at 16 than at other ages. On average.
That's not enough in my mind to justify the bro-science that all teens have insanely high test.
Within just 1 standard deviation is a huge variation in testosterone levels in teens and adults alike. While we should caution anyone considering PEDs, the basis for doing so should not be due to age related testosterone levels.