When I had my first DC, I was given a MS pregnancy and baby book. I had my DD very young, in fact by 21 I had 3 DC's. I bf them all, but even with my first, having successfully bf DD to 6 months, I stopped the day after, because the book told me there were no benefits to continuing.
As the book made it clear 4 months was the usual time to stop bf, when I had DS1, I stopped at 4mo. Then with DS2 I stopped at 3mo, because I had to go on medication that is incompatible with bf.
Fast forward to DS3, I had found MN when DS2 was about 1yo, so a further 6-ish years down the line, I knew that it was beneficial to continue bf for much longer, and MS was talking shite. I fed him till just after a year, when I had to go back onto the meds.
Even now, I wish I had bf HIM for longer, never mind the older DC's.
For reference, I had my first DC in 1998, so following MS wasn't that outdated.