Yes, if he does it frequently, it could cause her to start lactating. I'd guess if she allows it, she has probably had a foal before, just because I've seen youngsters try with adult mares & never seen one that hasn't foaled allow it, but I don't know whether that's a 'thing' or just my experience. I don't believe there are any medical worries, aside from if she does start producing for him, she will likely need some extra calories, then you wean him suddenly, she could get mastitis.

 

I read on this forum, putting yucky tasting stuff on the udder, like vicks vapor rub.

 

My younger gelding did this to my older gelding for a few months.
It wasn't so often and long enough for me to worry about it. It was also mostly light teeth rasping and licking, not sucking, so that was much safer for his skin. And would be fine for teats, if he was a she.
Is yours actually trying to get milk out by sucking or is the yearling rasping and licking? Don't stand too close and in too dangerous of a location when checking on that!

 

My younger gelding did this to my older gelding for a few months.
It wasn't so often and long enough for me to worry about it. It was also mostly light teeth rasping and licking, not sucking, so that was much safer for his skin. And would be fine for teats, if he was a she.
Is yours actually trying to get milk out by sucking or is the yearling rasping and licking? Don't stand too close and in too dangerous of a location when checking on that!

The mare shut it down a couple days ago so it was super weird but no milk. Now she hates the yearling and bites the crud out of him if he gets to close but there doesn't seem to be any reasoning that I can figure out. In all my years of horses I have never had to deal with this before but the mare figured it out so all good here!