WigWam, I will eat my NCT badge if you were told 16 weeks at an NCT class, truly. We have always echoed official guidance, which was 4-6 mths until 2003, though a few years before we might have shared with mothers the WHO stuff about 6 mths. I know I did.
It was never 16 weeks anywhere officially, but HVs did often tell mothers 16 weeks as a totally non-evidence based interpretation of 4 mths - ok a week will not make much difference to most babies, but weaning is a hassle, most babies really don't need more than milk until about 6 mths, and for some babies, a week will make a difference.
One problem has always been that some of HVs' training has been based on 'sitting next to Nellie' ie the approach whereby you learn your trade by watching what the experienced person is doing. Now, if Nellie is good and up to date, you will learn good and up to date practice. If Nellie spouts a barrel-load of cr*p about feeding and weaning, then sadly, you will do, too, unless you are able to challenge her because you have read some of the text books you are supposed to have read.
Occasionally, I am with a mother when the HV comes round, with or without a student, and I hear her say a load of nonsense and my heart sinks.