It's not reflexes. Demon's Souls has mostly puzzle bosses and the easiest bosses in the series. It's more a cerebral experience.
They actually kinda made the game harder in this remake, as I believe they limited healing items. In original Demon's Souls you could have ridiculous amounts of healing.
You can also summon, getting people to join you in defeating the bosses. You can get a few hints about easiest progression from fellow gamers, including good early items (most of the first levels have some good equipment in them). The bigger enemy will be the environment, which you'll have to explore and master, including the various enemies you'll meet.
Even the later Souls games, which have far more twitch-like encounters, can still be beaten without going overboard with reflexes. Honestly, your ability to notice patterns and be able to choose the opportune moment to strike. The game has been finished on like numerous different controllers that severely limit reflexes.
Say you don't have the patience, sure, but it's definitely not about your reflexes going down as you're older.