I wonder if it would be more accurate to calculate 5/3 of the baby's 12mo weight rather than 5X the birthweight.
All of my kids weaned well below 5X their birthweights.
DS was 8 lbs 14pz at birth and weaned at 4.25 (CLW) and at about 4X his birthweight.
DD1 was 7.5 lbs at birth and weaned at 2.5 (though I'm not sure this was truly a CLW). I think she was about 30 lbs then, which would also be 4X her birthweight.
DD2 weaned (accidentaly MLW) at 14mo.
But I can try to calculate how old she would have been if she'd weaned naturally. I can't even figure out from when she weaned off bottles because I switched her to sippy cups before she was "ready to wean" and then we kept up the sippy cups for bedtime water long after both girls needed to suckle.
She was 9 lbs at birth but only 18 lbs by a year. I don't think she hit 5X her birthweight until 2nd grade! 4X her birthweight probably happened around age 4.