depends on the nursery and your own feelings on the matter... and your needs.
Some nurseries take babies from a few weeks or months old, others only take them from 18 months.
DD only started nursery when she was 3 - before then, she was used to socialising quite a bit with children her age and we were quite active in the M&B groups. She'd also been spending some time (2 days a week) with a childminder in the UK when we were there. In the months running up to her 3rd birthday, she was quite obviously getting a bit bored at home, and I felt that it'd be good for both of us to get some space and extra, different stimulation.
DS will be starting nursery in September, when he's 2 and a bit, mainly because we don't seen to get to do half the activities with him that I used to do with DD... and I'll need some child-free time to do the final year of my MA!