Personally I don't allow it at all under any circumstances at any age. Everyone has their own idea and that is fine by me!
I don't allow nibbling, nuzzling or rubbing of any sort ever
and my reasoning is that all these behaviours, whilst very 'cute' under some circumstances, are sending the wrong message to the horse in my books, especially youngsters. While these behaviours don't start as overt disrespect, they pave the way for more disrespectful behaviours in the future.
If I am handling a horse, no matter what age it is, I expect it to be patient and not fidget, either with me or with the things I am holding e.g. Lead rope, brushes, keys etc. These types of things can be dangerous to them too if they are playing with something and accidentally swallow it. Very scary for the handler when you are finding yourself extracting a lead rope from your babies throat! Best to teach them not to fiddle with everything in the beginning.
If you want some bonding time with the little guy - YOU scratch HIM. He will enjoy it and it gets him used to the feel of your hands all over him.
Sure at a young age they like to experience things by popping them into their mouth but they are free to do that to their hearts content on their own time in the pasture, not when I am with them!
ETA: Butt in the dirt my comment about nibbling lead ropes was not directed at you, we just posted at the same time