That's a good question, Fuzzy, and there have already been some good answers which I hope have helped you
I agree with Sept Morn's definition of 'support' and 'assertive caring'. It is easy to support somebody. All you have to do is agree with what they say, and voila, the person feels better. However, the support has to be *effective* - ie the person has to benefit from it. A person can be supported in many things, but the whole idea of support is that a person will feel stronger from receiving support and will ultimately emerge better equipped than before. Some support can be damaging though. You could support somebody in their plan to kill somebody or their own body. You could support somebody in, say, making a decision which might result in bad consequences. So the person giving the support has the potential to 'make or break' somebody - though any decision is still up to the person being supported.
However 'assertive caring' is, I would say, when you step in if you think the person being supported is perhaps 'straying'. For example if the person is severely paranoid and thinks the police are spying on their every move - you could point out that it's not so likely, what have they done to warrant such behaviour by the police (if the person is completely law-abiding), etc - whereas in supporting somebody you might just listen to them and not really challenge their views.
I had never thought of it before, but it really is a fine balance. 'Assertive' caring is, I should think, far more effective than just supporting a person. However assertive caring is far harder in some situations, such as online, simply due to the distance involved - not knowing the person involved, not knowing the full circumstances, nor their past history, nor the context and other minor things like other people involved and their characters. So it is easier to give support online, but still possible to give assertive caring - but I think it is less likely to be effective since there is less likely to be a personal bond between the two people (since they do not literally know each other) and so the caring may not be fully beneficial to the recipient. There is also more potential for misunderstandings online too.
How do I think the nurturing of adults takes place... well I think the ideal situation would be for both to be in a stable relationship (not necessarily with each other) and that both would be able to effectively 'assertively care' for the other - they would be able to point out each other's flaws in a kind manner, so the other wouldn't feel humiliated but would feel able to tackle the problem. The other person would give *support* in aiding the other person, but basically the onus would be on the person working out their problems and doing something about it, feeling strengthened by the other person.
Co-dependency, from my understanding of it, is when a person no longer is fully independent - they come to rely on another person, seeing them more as an extension of themselves (as, I think, my relationship is with my father). So it is hard to be fully independent. Both rely on each other to fulfil their needs of support, and encouragement, but it doesn't always work because they are bound too closely together emotionally and it is hard to achieve sufficient emotional distance to change behaviours and habits. A person's behaviour is modified both by the other person's behaviour and by the other person's anticipated behaviour, so much so that it restricts their own emotional and intellectual freedom. Emotional development tends to stagnate and there is little experiment outside the co-dependent relationship in developing healthy relationships because of the consequences it will have on the relationship. It is altogether binding and hard to shake free of, because the other person resists the change in the relationship.
So when is the line crossed? It can probably not be applied to all relationships as a rule because it's handling different variables. However when I suppose you come to rely on one person to supply many needs - friend, confidant, etc - it becomes less of a 'two people and one relationship' but more of each person relying entirely on the other, with non-conformism discouraged.
This is my view on it anyway - and it's possible I have not understood co-dependency in its right meaning. I hope what I've written helps anyway - it's certainly long enough!