According to my naturopathic physcian, who specializes in pediatrics, women's health, and is a lactation consultant and a doula the gradual weaning process starts as soon as solids are introduced. For us that was around 7 mos. for my DS. She emphasied gradual.
Our DS is almost 14 mos now, eats 3 whole foods meals per day, and is still nursing at least 6x/day (down from 10+). Sometimes it's for nourishment, but often it's a quickie. We started teaching him sign language (ASL) at 6 mos. The first sign he learned was for nurse. He also signs for drink (water), eat, more, and snack. When he asks to nurse I figure he needs to for whatever his reason so I don't refuse him. He still wakes at least 1x during the night to nurse - sometimes as much as 3x depending on his needs.
The naturopath told me that according to her research, it is not uncommon for children to be emotionally and physically ready to self-wean sometime between 18-24 mos. They will let you know. But she also said that they still may want to bond-nurse before bed or when they first wake up in the morning.
My female in-laws, one of whom is a psychiatrist and another a child psychologist and their husbands make off-handed remarks about my breastfeeding. I've mentioned our physcians research findings. But I've found the best response is to say, "DS started weaning at 7 mos. and it's going wonderfully." They ask me the same question in numerous different ways and I just smile and respond with my mantra.
I believe that when it comes to nursing, children know what they need.