We need to be careful not to confuse correlation with causation. If an adult hasn't yet learned rhythm, he/she most likely won't. It's possible that such a thing is more easily learned at a young age, but it's also possible that the individual self-identifies as being rhythmically challenged, and/or doesn't have the time or environment necessary to support the learning process, and/or doesn't have any real motivation.
I have seen students learn the "Steve Jordan Michael Jackson" groove thing, in person, and more than once, and I have also successfully taught drummers who had trouble just "feeling the beat". The vast majority, however, don't learn, not because it's impossible, but because they have already decided to not learn, and/or because their life circumstances don't permit it. Strangely, one student hears the "Steve Jordan Michael Jackson" groove thing, and has learned to play it a little, but just doesn't like it, saying she prefers quantized, evenly-spaced perfection. Until there's a paying gig or band she likes that demands she play that way, she probably won't learn it.
Neuroscience is still murky on the subject, but rhythm and musical detection is located primarily in the cerebellum, which evolved much earlier than other "higher" brain structures (cortex, etc.). So, it makes more sense that the capacity for rhythm exists in everyone, including primates. Indeed, patients with disorders affecting the higher structures (Altzheimer's) can usually perform on their musical instruments without any problem, even though they might not remember learning the instrument.
If your childhood and family life did not regularly and socially involve dynamic, challenging, interactive musical experiences (singing, dancing, playing instruments) on a daily basis (the reality of most musicians), then you will not have developed the capacity at an early age. For a child it's somewhat common to have this musical experience, but how practical is it for an adult to engage actively in this way for hours every day? You can't fairly compare the results of an adult taking lessons once a week to the child who has had music, held in a variety of contexts and activities, for many, many hours, all of his/her life. The learning we do when we're young is immersive and challenging and regular; the learning we do when we're older, well, we barely have time for it. It makes more sense to blame life in general, and not the individual or biology. Besides, as a teacher, it does no good to start labeling things as impossible to learn.