I will say how it was life-changing for me getting diagnosed as 39M.

The quick answer:

Understanding neurobiology and the meta-cognition about the why of certain behavioural patterns and experiences is extremely helpful to just calm down and "forgive" myself, and not judge myself so harshly. It's as if I can optimise my life by leaning on my strengths and importantly recognising and dealing with my weaknesses. Just the awareness lets me catch myself getting into patterns, and act on them sometimes, and just let myself be the other times without any guilt. This insight brings peace more than you think. I can now organise my inner dialog and enjoy the diverse thoughts while also being aware of my impact on people in my life.

I was skeptical of the meds myself. I spoke with about 20 people through AHD support groups and group sessions. The drugs are mostly a trial and error affair with side-effects and altered behaviour. My subtle lifestyle changes - keeping one true todo list, always logging down my thoughts and taking comfort in the fact that they are jotted down so that I dont have to keep going down that thought and actually get up and go catch the train etc already gave me the biggest benefits. Then I wanted to see for myself what the drugs could actually do. At the end of it I may choose to take them or not. After 8 months of slow analytical trial and error with a very helpful psychiatrist who understood where I am coming from, I now take a very small dose of Elvanse(Vyvanse) + Guanfacine on some days of the week where I feel like I could use the help. They are not life-changing. They nudge me into being more capable of being me. My wife can notice the regulation the drugs let me have - A win for me. My kid has a better relationship with me than before - A win for me. Could I do without? Sure. But, I choose to have that little support to manage myself to get to who I want to be around my family without losing who I am. To each their own.

Longer background:

Despite reading about and knowing the symptoms of ADHD, and being told by a professional 10 years ago that I should consider getting formally diagnosed, I thought to myself - "Hey, I am smart. I feel happy and satisfied. I dont have any problem. I'm good as I am. I dont need to do anything about it." and promptly kept sweeping any thoughts about it under the proverbial rug.

Until, I started seeing my son develop in front of me. Seeing all those symptoms - impulsivity, hyper-focus on few things and inattentive to most others, emotional regulation issues, extreme reasoning abilities in some areas while being "immature" in other areas. This was me growing up. Being smart and intelligent, but longing to fit in, struggling to deal with the mundane. So, I decided to get diagnosed properly. Basically, I could hide behind my smartness and the ability to manage my symptoms until it just myself in my life, and then my wife, and then my son, but when the second one came along, I couldn't manage anymore and the overwhelm started to show the real patterns.

It takes courage to be honest and vulnerable.