For me it was when I start noticing that the bad days where I'd fail my intended lift happened more often than the days I progressed.
An example, when I was trying to push my bench press over 87,5kg I was going with 1-2 sets of warm-up (~8-10x 70kg then adding 5kg to the 2nd warm up), followed by: 1 set of 5x85kg, 5x87,5kg and trying to go 3x90kg but noticed I was failing the 3x90kg set for 4+ weeks in a row, I kinda knew it was going to take more effort and a different approach than just my very basic way to increase load, I was happy with lifting 87,5kg on the bench and just stopped trying to push it further.
Age has definitely affected my recovery time, stamina, etc. but because I've been lifting on-off since my teens playing tennis, and only focusing on strength training for its own sake later in my 20s. I do have a "baseline" that I can reach in about 4-6 months even when I stop training completely for a while, the strength you gain changes your muscles (I think it creates extra nuclei in muscle cells, unsure how true that is, lots of pseudoscience in fitness-world) and it comes back. That's one of the main reasons I recommend all of my sedentary friends to try lifting for a while, it will help when one is older.
An example, when I was trying to push my bench press over 87,5kg I was going with 1-2 sets of warm-up (~8-10x 70kg then adding 5kg to the 2nd warm up), followed by: 1 set of 5x85kg, 5x87,5kg and trying to go 3x90kg but noticed I was failing the 3x90kg set for 4+ weeks in a row, I kinda knew it was going to take more effort and a different approach than just my very basic way to increase load, I was happy with lifting 87,5kg on the bench and just stopped trying to push it further.
Age has definitely affected my recovery time, stamina, etc. but because I've been lifting on-off since my teens playing tennis, and only focusing on strength training for its own sake later in my 20s. I do have a "baseline" that I can reach in about 4-6 months even when I stop training completely for a while, the strength you gain changes your muscles (I think it creates extra nuclei in muscle cells, unsure how true that is, lots of pseudoscience in fitness-world) and it comes back. That's one of the main reasons I recommend all of my sedentary friends to try lifting for a while, it will help when one is older.