As far as pure preferences go - my anecdotal data is it can vary widely, and isn’t necessarily sex-linked. So in our case:
Daughter #1 - gradually developed preferences at about 5 or 6. Before that, clothes were things you put on yourself so mum and dad would let you outside to play.
Daughter #2 - decided overnight she wouldn’t wear any colour but pink, at age 2. Remains very girly 2 years on.
Son - started developing very decided clothes preferences at about 20 months, mostly based on function (eg gumboots so he can jump in puddles … it may have been a week since the last rainfall, but he lives in hope!) but also on the colour red, which is always a winner. Complains VERY loudly if put into the wrong clothes.
As far as being aware of what other people think of their clothes - I don’t think any of them are at that stage yet. The Taller Girl just likes things because she likes them - she’s equally happy putting me into nice clothes as herself, or creating a nice picture in her favourite colours. I may have a different take on it when the Smaller Girl reaches school age, however.
I imagine there’s a big socialisation component. I don’t pay a huge amount of attention to clothes, so there’s not a lot of pressure on my own kids in that way.
Looking around, I would say 5 or 6 is a good guess for a minimum age of “clothes-consciousness”. I remember a lot of my daughter’s preschool friends had very definite opinions about their clothes, but it was all very individual - boys who turned up in their batman outfits week after week, or little girls who wore a tutu all year. School seems to be where the peer pressure in general starts to kick in.