Well, reading the article states that there's a slim chance of the vaccine causing myocarditis, and a source linked to from wikipedia [0] (don't @ me, I'm no scientist) states that young people are more affected by myocarditis [1]

[0] https://en.wikipedia.org/wiki/Myocarditis

[1] https://books.google.nl/books?id=-nIpAgAAQBAJ&pg=PA135&redir...

The recommendations are simply based on estimates of the observed incidence rate by age.

We don't even yet know for sure the mechanism by which the vaccine can cause myocarditis. There are various theories, a number of them related to the spike protein itself.

Figuring this out should be of the highest priority given that public health officials around the world are basically moving towards a boost every 6 for the foreseeable future position.

There's a lot that changes in your body as you age. One that seems to be important for covid is that your immune response gets weaker as time goes on.

Myocarditis is inflammation of the heart and is believed to be caused by the immune system's response to the virus. It maybe that it's less likely to happen in 30+ because their immune response tend be more muted.

Younger people have a stronger immune response to things. Could just be an overreaction by a younger person's immune system.

I'm not sure of any direct scientific evidence supporting this, but I believe it has to do with the vaccine entering the blood stream.

The serum is meant to be administered at an intramuscular site, but injections have a chance of hitting a blood vessel upon insertion.

I am not a medical professional, but the advice I've read is that practitioners should "aspirate" the needle (pull back on the plunger) before delivering the vaccine, to see if they've hit a blood vessel.

https://www.youtube.com/watch?v=KgVsd6qoyU4

This theory somewhat matches the observations because young people are likely to have more, larger blood vessels in their arms compared to older demographics.

This also explains why men experience myocarditis more than women.

Could be an increased efficiency towards a negative outcome. Older peoples bodies maybe have different uptake of the serum. Maybe that means there should be a spectrum of dosages depending on specific characteristics of the person, like weight, age etc?

The immune system is different and reacts differently with age. In other words...the fact the older aged persons have less of a risk with the vaccines is because their immune reacts...(dare I say it?)...reacts less?

"How Aging Affects Your Immune System" https://www.webmd.com/healthy-aging/guide/seniors-boost-immu...

I don't know about this specific case but previously (e.g. with AZ) the concern was not that a specific vaccine had worse side effects on younger people.

It was a balance of the risks of the vaccine pretty much in general versus the risks of Covid by age range.

So basically if the risk of Covid is high it makes to get any vaccine available. But if the risk of Covid is very, very low then you may start considering what the risks of getting the vaccine are.

A guess: The benefits of the vaccine to a 35 year old are substantially larger than to a 25 year old, and Moderna seems to provide better protection to new variants than Pfizer. This may be mostly just because the mRNA dose is larger in Moderna than Pfizer, but with a somewhat worse side-effect profile.

In other words, if you have a medicine with both risks and benefits increasing with dose, the optimal dose will generally be larger for people who stand to benefit more.