Is it possible they've just shown that older transgender people are unhappier than younger ones?
Haven't got time to look at the paper right now, but wondering have they differentiated between MTF and FTM
All participants were pooled for analysis, with (buried in a set of table footnotes) adjustments to analysis models for age and a set of other potentially confounding factors, including (for most analyses) biological sex.
I think a cautious interpretation would be that, compared to a younger group of trans people who at some point in their lives wanted cross-sex hormones but never took them (for unknown reasons), an older group of trans people who started cross-sex hormones in adulthood demonstrates some better outcomes (current severe psychological distress, recent suicidal ideation) but also some poorer outcomes (lifetime suicidal ideation illicit drug use, and current alcohol abuse), for reasons which are unclear.
What is clear from the study is that starting cross-sex hormones post-puberty doesn't result in overwhelmingly better mental health for young adults. Although the data from the groups starting hormones as adolescents appear to show better outcomes, the numbers in these groups are small, the outcome measures have a limited focus, even if the differences are real, the data don't prove that cross-sex hormones are the explanation.