I know that horses will lick where there is rust. Some just like the taste of it and I've heard that they will lick rust because they are lacking a mineral. I can understand them licking us for the salt when we sweat.
Our Ghost licks the hay manger after eating, the metal gate when we take him back out to pasture, your hands, arms, clothes, even your face if he can. He never bites. He likes to play with his bowl after eating too. He'll hand you his bowl if you're standing there or he'll throw it on the ground to let you know he's done and wants more, lol!
I know he wasn't weaned early. It was at 6 months old. Our TWH was weaned around 6 or 8 weeks old but she doesn't lick at all. They have a salt/mineral block in their pasture. It must just be his 'thing'.
The other horse I knew that did it, I had only seen a few times, but she would lick everything too. Both are Appys and they both are about 5 yo. Maybe it will go away as they get older. It doesn't really bother me. I just think it's amusing.