I think what people mean is that you do not hit any age and suddenly have a different body. There are changes that occur over time, after injury, birth,etc, but as a seasoned 45 yr old I can honestly say it’s a matter of perception.
For instance, my arms are lean, thin, my waist tiny, but my legs have altered somehow in a way which I can’t quite define yet. My boobs are l lot less buoyant too!
So we all look differently, at every age. By the logic of big arms/belly, this could also be applied to a teen. It’s just institutionalised body loathing. Insidious, and shite.
My advice is to forget the age element completely
Focus on how you ‘feel’.
Make changes from this basis, and consider your current lifestyle.
Then clothe yourself for that.
Figure out what ‘skinny’ means to you, and put it to rest. It has nothing to do with health or beauty. Aim for a healthy body and fitness, if you don’t do gyms, try an at home YouTube toning vid, there are plenty recommended on here. I’m slightly underweight at the mo and definitely want to do a joe wicks YouTube stint to build up a bit.
What you want to change, dress-wise, depends upon what your current wardrobe is like. Work out what makes you feel most comfortable and stylish, and reject what doesn’t. It sounds simple but it often isn’t!
If you prefer to minimise elements of your physique, choose items which make you feel relaxed/good. So perhaps swap cap sleeves for 3/4, etc.
Don’t force yourself to follow style whims or rules that make you less than confident. For example I have a short torso so high waists make me look frumpy, even at my low weight, as my boobs almost meet the top of a high waisted skirt. So now I go for mid rise, etc.
Just make a true and honest analysis of what you wear and why you wear it. Adapt from there. But forget age, nothing at all actually happened after 40. I am just about in peri meno right now and nothing wild and weird has happened yet apart from a very slight redistribution of weight (more in thigh, less on waist/up top) and somewhat unpredictable periods🤪
Good luck.