i read that this an evolutionary adaptation - that kids are actually designed to get more careful/picky at the same time as they get freedom of movement and the ability to access more different things to eat.
back in caveman days it was to stop them just wandering around the forest and sticking anything in their mouth - i guess like poison mushrooms or whatever. they would just stick to the roast woolly mammoth that they 'knew' was definitely safe.
if this is true, they should grow out of it as they get the brainpower to understand what is dangerous to put in their mouths, so maybe age 3 or 4. that has been pretty much my experience with my daughter - she got very conservative about food from about 18 months, then more adventurous from about age 3.5 or so.
the other thing my mum who is a doctor always reminds me is that babies grow physically a huge amount - double their weight in a few months etc - but toddlers growth is slower and is more about developmental ability. so it can SEEM to us like they are suddenly eating 'less' or 'not enough' but they have tiny tummies and suddenly need less physical growth from around 18 months.
i agree with the 'you decide what & when, they decide how much'. my little one definitely hasn't starved from following this idea, so i am happy as long as i offer her healthy food to just let her stop when she feels full.