The perfect is the enemy of the good.

24 is 6 extra years of maturing. That will save a lot of lives (not just mass shootings, but gang violence and suicides). And 24 better reflects reality in terms of adulthood.

Not only that, but it's palatable politically. As long as it goes with voting age and other rights, then gun owners are more likely to go along with it. Make some exceptions that work for young hunters, and people won't feel left out.

If anything, it's likely to see pushback from the left because they want lots of people voting from a young age. But I don't think the overall electoral decisions will be any worse by making people wait 6 more years. The left will just adapt and move on (e.g. more focus on struggling families and less on cultural/identity issues).

25 is widely accepted as approximately the age for full brain development, on average. It's part of why car insurance rates tend to drop around 25.

My point is that a 35-year-old person might have mental issues, that's why I’m saying that it should be checked.

Which psychological tests specifically? In general there is no test that can reliably quantify an individual's propensity toward violence.