Youngest went at four months for three half-days a week because she seemed to be very fed up when her toddler brother wasn't around to amuse her!
He went at about nine months, for three half- days.
Gradually both of them built up to four half-days a week. It worked well for them and me, as I could work part-time+ (using nights to do some more and also 'swapping' children with other friends to gain more hours).
Even from being a newborn, the youngest has always been very happy in busy places with lots going on around her. Oldest, not so much, but I think their really lovely nursery helped him with this.