Oh my goodness!!
No! Don't ever discourage this beahviour. If you can stand the feel of it, that is. Suckling and/or kneading has nothing to do with how early or not they were taken away from their mom. I have had a kitten that was ripped away from her mother before she was 5 weeks old. Never suckled or kneaded anyone or anything. And I have had cats that were with their mothers plenty of time and have done just the opposite! This is a regression of sorts to when they were babies. They kneaded their mother's stomachs to get the milk and then of course, suckled the milk. This is a highly trusting and loving act. If your furbaby thinks of you so much as his/her mom, then you should feel honored! To turn them aside when they are extending this much trust is a rejection. I have read up on this because my Tiger used to like to come up every night and lie next to me and knead my arm while he suckled my shirt and purred and purred. This was one of the only times he truly purred. But you can well imagine the state of my nighthsirt after awhile!
Then when we got Ashke, my husband used to have to hold him every day, about 3 times a day while he suckled his own paw.
And then he would come up every night and lay between us and do the same thing while we petted him. He outgrew being held on his own, but he still comes up to have his nightly sessions with us. But he now only suckles about 2 - 3 times a week. I guess sometimes they do outgrow it. Our Aiden will knead us some and purr before he curls up to sleep on us, but he never suckles. Yet he is still so friendly. Every cat is different, I guess, just because they don't do this doesn't mean they don't love us just as much. But please, never reject this bonding between you and your furbaby.
P.S. I volenteer at a shelter and have had cats over 10 yrs. old knead and suckle me because they were just so happy to have some human love and affection!