I do respect personal choice but I don't understand why a relatively young woman would allow herself to go grey.
So... you 'respect' personal choice, but then go on to criticise it. Women don't 'allow' themselves to go grey; it is a natural function of the body. Some women naturally go grey in their 20s, others not until much later on in life; so grey hair is not always an indicator of age. 'Grey' hair has a massive range of colour from pepper-and-salt, to stunning silver, to snowy white. I have a neighbour who went grey in her 20s (now in her 50s) who has a waist-length mane of stunningly beautiful silver hair. It is distinctive and much-admired. It actually makes her seem age less, since her appearance has not altered for years.
Hair colour is a simple anti ageing tool. It's easy to do, cheap, non invasive and doesn't require surgery.
I disagree. Once you start colouring your hair you are stuck on a hamster wheel of regular salon appointments, expensive dyes, tints, highlights, lowlights and root touch-ups. Home dye jobs are cheaper than salons, but still not what I would call cheap and still require regular maintenance. Plus. they are a faff to do at home and it is all too easy to accidentally dye clothing, towels, etc.
Why do some women want to be grey? I don't get it.
Well, clearly you don't get it. Maybe that's because other women are not so ageist or judgemental? What if you had an allergy to hair dye and found yourself having to let your hair grow out grey? And then heard your friends commenting on how old you looked? Especially when it would probably be tosh, and more to do with their just not being accustomed to seeing a grey-haired woman. And maybe that is all about a society that prizes youth and tries to squeeze all women into a conforming ideal of beauty; coloured hair, make-up, false eye lashes, shaped and tinted eyebrows...the list goes on. Maybe we all need to start recognizing that women are beautiful in many different ways, at all ages, and we should empower each other to express ourselves however we want?