ADHD isn't something you suddenly develop at 30 or 40. You can get diagnosed later, but the signs are meant to have been there since childhood.
Alongside the siblings, the types of challenges you face as a kid and as an adult are different. You might be able to manage schoolwork on natural ability alone while your parents and other adults are your "life personal assistants" that take care of a lot of things for you, but fail when it comes to managing adult responsibilities yourself, or maybe being the "life personal assistant" to a child of your own.
Symptoms become more obvious as you age, which can feel like it came on later. As part of an assessment they do go through your childhood - if you had trouble in school for example that's a classic ADHD symptom, but there's a lot more to consider. It's not always obvious and I've met plenty of people in their 30s, 40s, 50s who only just realized they had it this year and got assessed.
Glyphack's experience definitely mirrors my own and I just got diagnosed with ADHD this year and medication absolutely helps my focus.
If you're wondering if you have ADHD this is basically the pre-assessment GPs use in the UK (which then continues to triage and then a full assessment):
ADHD is something you can rapidly develop an inability to manage at that age, though — after trauma or burnout. Or maybe following RTO after a couple of years of a more peaceful WFH environment.
You can get through school without failing, you can get through university and early jobs with hyperfocus, you can be known for being the person who gets bored easily but can do incredible things. Maybe you are late paying bills, never get round to doing certain big tasks, rent when you should be looking to buy a house, all that. Maybe you have friendships that never quite develop into romantic relationships, maybe you are untidy, maybe you stay in a lot because you fear you tire people out socially. But you bimble on and you convince yourself it's OK.
My ADHD burnout started in my late forties. Before that I had any number of explanations for the unusual, eccentric aspects of my life, for why I was a freelancer rather than in a steady job, all of that. I still managed to get big jobs done with relentless bursts of focus. I had already quit social media, I had already learned to manage some distractions, and it still happened.
But it's clearly ADHD. Having read about it, "inattentive" (hilarious name for it) ADHD explains me to a shocking degree; I don't need a diagnosis.
I just never failed badly enough that the issue was forced, until now.
I burned out to the point that it feels like cognitive decline. I am broken and I struggle with everything. What the writer is talking about is so reminiscent of the early stages of my burnout that I worry for him.