There isn't any 1 answer to it. But if you live on a diet of white bread cheese alcohol. And lots of sugar. I would expect it to begin when puperty ends (maybe even before)
For me i know that after really hard physically work. I get some intense hard boners.
Remember one time i brought this chick to a hotel. And i had been training up to a fight and i had given it everything.
So we get lost in a moment of lust and i rip of her clothes and lifted her up and fucked her while she was hanging on me.
I banged her like this for 15 minutes with a rock hard erection. We stopped because she got tired in her arms.
Can i do this today? Hell fucking no! What im trying to say is this.
Your penis is a part of your vascular system. By improving this. You imrove erections.