We did not send our dc to nursery until they were about 3 years old.
We have 4 dc - the oldest two boys are now at pre-prep and prep. ds3 is still at the nursery and dd will start there in January 2012.
We did a lot of research and visited lots of nurseries before we settled on the one we now use. Our criteria was that the nursery had to be well thought of by parents past and present. DH and I had to have a good rapport with the staff, and we required that they had a 'door open' policy for parents wanting to discuss their children's development; the teachers also had to be good, kind people.
We prefer traditional, academic teaching in a relaxed environment. The teaching/fostering of social skills, including kindness, empathy and politeness were also a must. And we found a great nursery in central London which ticked all the boxes. And so far, so good. Upon leaving to go to pre-prep at 4+ years old my two oldest ds were very independent, academically able (reading very well at 4 and doing good basic maths), and enjoyed the creative subjects.
ds1 was incredibly shy when he first started and nursery worked with DH and I to help him gently along with his confidence. By the time he went to pre-prep he was playful, happy, confident boy. And I know that it is because of the wonderful guidance by his nursery teachers. The Headmistress gave us great advice about other schools to consider as son became older.
In a nutshell I adore our nursery and the staff are fab, they have provided a safe, nurturing environment for our children to develop.
I don't think that I could have sent them to nursery earlier than 3 years old. We went to playgroups paint pots and playgroups at our local library etc when they were smaller.