When Lauren (now 6) was little the recommended weaning age was 16wks, she was 14wks and the Health Visitor recommended starting to wean her then.
Abi (now 4) was about 15 and a half weeks.
Toby (now 11 months) was about 16wks, even though the recommendation is now about 25wks. Again, my Health Visitor advised me to start weaning him early as he had started to wake in the night etc after previously sleeping through.