I am going to be 42 this year end and went through similar motions last year. I almost thought of calling 2018 my last innings.
The reflexes certainly go down in our age group players. But as RF pointed out these can be adjusted with practice to improve the necessary skills needed to field and complete a catch.
Few things which I did start at the beginning at this year were:
1. A gentle but regular strength training with special focus on shoulders. My rotator cuff was giving up last year, but with little physio assistance/guidance I am now able to throw and rotate my arm more freely (to bowl) than last year.
2. I got involved in badminton and indoor soccer in the off season (co-ed mixed level group), and it did help in my endurance & improving aerobic capacity. Of-course, I lost few pounds off the belly as well and I can run around bit better now.
Both these tricks worked for me in improving my overall cricket playing abilities, but more significantly had a positive impact on fielding. But these two tips work only towards setting goals for the BODY (Body, Mind & Soul). Fear of dropping a catch and its aftermath is the most challenging to deal with. Trust me, I followed exactly as RF mentioned "Try and keep your head still when you catch. Confidence is key though, if you think you're going to drop it you probably will. Everyone drops catches by the way".
Just to mention, I have played 12 games (all T20) so far in this season. Five catches came way, out of which three were taken at point, square leg & one at deep. The dropped ones were at mid wicket where I felt that the bowl came in lot quicker & flat. I felt that my reaction time was not there. Will avoid these positions in 2020. In 2018, I had dropped all the four catches that came my way.
In summary, put in a little effort towards improving strength & fitness but do bit more practice towards working on those fielding skills. Cheers!