I was still getting ID'ed for ciggies/ale until I was about 35 so yes, I know how it goes. I couldn't blame it on my height, either- I'm 5 foot 8. My face just looked 'babyish'. At the time, I lived in a city where the 20 year old girls' looked like my mothers' age, and I never really appeared 'polished' like they did. Couple that with my lolloping walk and 'quirky' art student fashion sense, and I used to seem a lot younger than I actually was. I've done my stint spluttering in Tesco's because they won't allow me to buy alcohol because I don't have a driving license and don't have my passport with me.
If it's a confidence thing, then, I reckon you can fake self-confidence, to a certain degree. A lot of it is about 'presence', or 'taking up space'. Women are often programmed to be 'girly' and apologetic, and eager to please, and all that girly makes us look so young. Standing tall, sitting up straight, using power gestures such hands on hips, clasping your hands behind your back when you're talking, walking with purpose, not much talking, practising your resting bitch-face, not giggling or covering your mouth when you laugh, etc, not being so smiley and 'nice', reducing your facial expressions to the bare minimum: all of that can make you seem less approachable, more stern, and more naturally 'mature'.
If you want to look even older, then think about how your appearance intersects with your posture/attitude. You can go wild on nights out, sure, but, day to day, maybe try a more 'classic' and 'tailored' style for your daily 'uniform'. Think 'professional', and 'businesslike', but obviously, not too try-hard. No need for, say, a briefcase and tailored shift dresses and high heels, unless you really do work in a corporate-heavy environment. Think more along the lines of... posh junior doctor, than secretary/office-wear, so... trench coat or woollen overcoat instead of Puffa jacket with a furry hood, a collared blouse/shirt instead of a raggy band t-shirt, a crew neck jumper instead of a fluffy cardigan, tailored trousers instead of Mom jeans, that kind of thing. Simple jewellery, no wild hair colours, no 'trendy' make up looks, always lipstick, short nude nails instead of coffin shaped acrylics, no hoodies, no gym-wear, no slogans or logos, nothing cutesy or overly flamboyant. Sensible shoes if you're on your feet a lot, trainers are okay for our imaginary junior doctor, so long as they're subtle. Depending on your natural colouring... maybe stick to a simple, tonal colour palette, too. Pink and lemon and lilac, for instance, might just emphasise your youth, whereas a navy overcoat with a pale blue shirt, or a dark green and sage, or deep tan and cream/beige might be a more 'mature' seeming palette.
Of course, I'm no fashion maven, but those are the kind of things that worked for me when I needed to 'pretend' to be a grown up, and now I'm mid-40's I can wear whatever daft clobber I fancy: nobody's mistaking me for a teenager these days (sob).
Feel free to ignore my ramblings: hopefully something in there will be useful to you in your reinvention quest.