There is a law about it (having just checked ) - they cannot be marketed as suitable for under 4 months, and that is all that is stated. Prob as below 4 months definate harm is known to occur, and then its a bit fuzzy between 4 and 6 months.
Its only fairly recently that the advice changed from 4 to 6 months. DS1 is 4 years and when I was weaning him the advice was 17 weeks. Probably takes longer than that for advice to filter through to food legislation
They can change legislation in 6 months or less if they feel like it (have been on the end of that in my field), and require you to recall all your stock and relabel if deemed necessary. However, since big businesses are involved, they seem to manage to resist the change, just like the infant formula legislation for which they have forced a judicial review
The DoH have recommended 6 months since 2005, and before that the advice was between 4 and 6 months, not 4 months.
It could be that other English language territories (like here in Ireland) advise 4-6 months as weaning age for formula fed babies. They advise 6 months for breast-fed babies only, and I've no idea why it's different. I've notice more new labels going for "stage 1, stage 2' etc rather than just ages.
"Over the years the guidelines as to when to start weaning have changed dramatically. Your parents might tell you how they gave you rusk in your milk when you were only 8 weeks old. And if you have older children you may remember that mums were advised in the last decade to start weaning when their babies were 4 months old.
In 2003 the Department of Health issued guidelines (based on recommendations by the World Health Organisation and UNICEF) that weaning should start at 6 months.
It is now thought that babies under 6 months have kidneys and guts that are not mature enough to cope with a more diverse diet and that early weaning can increase the risk of infections and the development of allergies like eczema and asthma."