According to Kathryn Dettwyler, most mammals wean at triple their birth weight (which is one fact that has encouraged the 1 year weaning in the recent past). Primates are more likely to go to quadruple their birth weight which would be from 2.5-3.5 years for humans.
That said, Dettwyler says 1/3 adult weight is a good indicator, which happens from 5-7 years in humans.
Here is a quick rundown:
http://www.kathydettwyler.org/detwean.html
DD was around 5X her birthweight when she weaned and DS was over 5X his birthweight when he weaned. While I have heard the first molars are pretty predictive, they were both very late teethers (at 7.5 DS has lost two teeth, at 10 DD has lost 8) so they weaned before that. The baby teeth used to be commonly called "milk" teeth, but both DD & DS were weaned 1.5-2 years before they lost their first tooth.
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