
They still will nurse when given the opportunity. In fact if their mom is not around they will suck on another adult (even a male) or a fellow kitten.
When is enough ?
It is a comfort thing for the kittens but is it so for the giver? When it was happening to my older male cat Kingstone he was ok with it but after a while he was not too happy. I would imagine getting all wet and having younger kittens tackle you down cant be too fun. In time they will outgrow this behavior.
There is a time in all our lives when we just have to move on and out of our comfort zone.