If you practice then the age isn't going to stop you.

Look at this old guy. He still has it.

https://www.youtube.com/watch?v=NQtSdgVqQvo

That looks like years of muscle memory.His time have would have been slower been if he had to react and shoot at a moving target.

Are you sure it's a lack of reflexes and not a lack of concentration?
Just asking because if your reflexes are going bad you might have an illness that causes it.
It's a general misconception that reflexes get worse when you get older because if it isn't caused by an ailment it's a lack of concentration that causes you to react slower.

And fighting games to me have always been boring, so I'm not good at it and ergo I can't concentrate on it.

edit:
By concentrate I mean focus.
I thought it had the same meaning in English as in my language.

Well yea, death or alive V went pretty shitty all in all, had a good few months trying out some of the eastern styled martial arts available, i belief i managed to progress half way through the campaign but that was where live really became rough. Tried a couple of training rounds and there i ran into a similar problem as the op mentioned.
there is only so much room i want to dedicate into a game to try and get better at it. for example the eldest souls i bought earlier this week has a very big chance of joining the backlog without giving it a good try, same probably goes for Mortal Shell. Jedi : Fallen Order and Sekiro are probably games i will finish, and maybe Tekken 7 too, fighting seems to be just a bit less technical in those games

I think that problems with concentration can come with age as well. Maybe it's not a huge problem in comparison to reflex degradation, but it's there as well. Of course there are other things which influence concentration in a more meaningful way than age like noise, surroundings, thermal conditions and others, but the age is still there somewhere in the background. We get less efficient with age in overall, but we can also slow down the process with exercises. Reading and writing is a good way to improve your shape! :)

Jerry Miculek is another person that became known for fast shooting.

The targets I mentioned would be the kind that suddenly appear and start moving so they test more than trick shooting.

Ah, you and me both, StingingVelvet. I used to be quite ace at Sonic the hedgehog. Now I'd probably be dead if my life depended on my prior "skillz". Haha. I guess I've also grown to be drawn more to RPG, Strategy and Adventure games. You know, more on exercising your logic instead of your fingers and hands.