I wonder how true this really is if you make an reasonable effort to keep yourself in shape. It wasn't until I hit 60 that I felt unquestionably different, and even then it wasn't terrible.
At the time I was walking about 4 miles every day for years both uphill and downhill. Around the same time as me waking up exhausted, the walks became harder and harder for no apparent reason until I eventually just couldn't do it anymore. No doctor I've met has been interested in diagnosing why, because "lol you just need to get fit".
fao_|1 month ago