It's not about being the youngest.
In systems where all children start school agred 6-7 - when formal learning is developmentally appropriate - there is minimal measurable difference between the oldest and youngest in a cohort so it doesn't matter who is older/ younger. And none of these negative effects that persist throughout education and into adulthood (emotional, social, mental health) seem to manifest i.e. no significant difference between the outcomes for the oldest and youngest in the cohort when they start at that age.
But when they start at 4-5, there is. A very significant difference that then persists. It doesn't disappear when they reach 6-7. It narrows a little simply because there is less of an age gap so in the long term there's nothing to be gained by being "oldest in cohort": that effect vanishes. But in the UK system where they start so very young - before formal learning is developmentally appropriate - the data suggests that this does long-term damage. Because at 10, 15, 18, 21, there is still a significant difference that does not exist in systems where everyone starts later.
Therefore the issue is not their age compared to the others in their cohort because that effect vanishes over time anyway. The data indicates that there is a damaging effect based on the absolute age at which a child starts school if this is too young.
An effect which never unwinds. It's nothing to do with competing with others in their cohort. Who cares? In 20 years they'll likely only ever see one or two of these people again at most. It is about the long-term effect on the specific child from starting school when they are too young to be doing formal learning. NOT any comparison to their peers.
And while we're at it, their peers in the UK who are 5 in Sept are also being damaged by starting too young, but just not to such a great extent as the ones who are 4. Hence them being outperformed by children in other school systems who start at 6/7 when it's more appropriate to be doing writing etc rather than playing and using their imaginations.