It's old enough to heal more slowly, but also experienced enough to not care as much about pain anymore, cause it's an old friend that's pretty much permanent. It's old enough to be cautious and wary, but experienced enough to end things brutally and quickly cause long fights are more dangerous to us.
It's old enough to have seen pretty much all the tricks and experienced enough to remember all the holes we had at a younger age.
And when we look in the mirror, we don't see what you see when you look at us. Speaking only for myself, I'm convinced I'm still that guy, only without the self-doubts. Brother, I know who I am and what my limits are. It helps to know what size ring you're fighting in and how much is left in the tank. These things one learns after life has had you by the short and curlies for 50 years or so.