The thing is, when someone suggests co-sleeping, and then goes onto say that they're still doing it with their DC aged 4, it can really freak people, who the fear that it'll never stop, out... :(
I co-slept with mine when they were tiny but around 4 months of age, started settling them into Moses baskets next to my bed with me touching them until they slept, and then without the touching, and then eventually, post 6 months into their own beds - all a very gradual process. So I have no issue with co-sleeping.
But for some people, there is a very real fear that their bed and their, um, conjugal relations will be foresaken for years and years by taking the co-sleeping route.
There are so many threads here written by sleep-deprived parents at their wits end asking how they get their baby or more likely toddler to self-settle and to sleep for longer periods (which absolutely requires the ability to self-settle), and to my mind it makes sense to do it earlier rather than later. And by earlier, I do not mean leaving a young baby to CIO.
I am all for co-sleeping, as I say I did it myself (as a breastfeeder, it is sort of part and parcel), but I have to be honest and say, I would just disregard such advice from someone who was still doing it at 4 years of age, as that would be my very fear. Just sticking up for coraltoes a bit.