at 43 you have a good decade left..

for me the decline from 25-50 was negligible. It was mostly due to increased life stress and time demands leaving insufficient time/energy for adequate training and recovery. There was also an episode of cerebral malaria at 28 which knocked me back for several years, and didn't ever fully recover from. All my run PRs are from pre-malaria. That's when I took up tri seriously ;-)
I set my Olympic distance PR at 47, after taking some years to learn how to ride a bike.

After 50 the decline started, after 60 it's a cliff..
for me it was cascading injuries, each requiring time off to rehab, after which the rebuild never gets back the pre-injury fitness. Also, sleep quality declines, and recovery times increase. At the same time as we need more training hours to retain fitness, the recovery from that training becomes more difficult.

my coping mechanism is to practice gratitude, though it's much harder than training ever was..

times at a local 5k, from age 50 -
2010 20:31
2013 22:37
2015 23:25
2018 24:27
2019 25:50
2022 26:45

The 26:45 represents six months of focused run training, was 28 or so at races in 2021.
2009 was the last time I went under 20min, PB of 15:50 in the 80s..

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"It is a good feeling for old men who have begun to fear failure, any sort of failure, to set a schedule for exercise and stick to it. If an aging man can run a distance of three miles, for instance, he knows that whatever his other failures may be, he is not completely wasted away." Romain Gary, SI interview