Was expecting a smart ass answer like that. Obviously age is just a number . Just look at Bernard Hopkins.
The first sign is when people on forums start saying **** like: "Fighter A' doesn't need to fight 'Fighter B' at this point in his career, he needs the big money fights.
Think probably when speed or the amount of punches they throw starts slipping. Very difficult to gauge those things fight by fight though, only really becomes apparent when you look at the bigger picture.
If you're Shane Mosley... the Hand shaking. It showed that he was reluctant, if not unable to pull the trigger because he didnt trust his speed anymore. Edit: by hand shaking, im not talking about the nice-guy, high five crap between rounds...i mean him actually shaking his fists.
I don't think it's the same for everyone. But a common one I think is not being able to pull the trigger.
Legs and speed. RJJ didn't get KTFO because his chin deteriorated, it is because his speed was gone that enabled him to be unhittable on the button.