Just whenever they can sit well enough. There is no exact age. Both my boys could hold their heads up from birth, so were able to be in the sitting position very early, but every child is different. If they can sit supported without their head rolling forward etc, then they are ready.
Oh I'm so glad to hear somebody else say that!! When unsettled one of the only ways we can get Euan to settle is rocking him in the pram - but for some reason he'll only settle when nearly upright. After he's calmed we can slowly start to recline him but we can only get him horizontal once he's already asleep (whether in the pram or cot) - he is the most insistently upright baby! I've been worried that we are damaging his neck but he seems to have very strong neck control, even at such a young age.