I think I was around 19 or 20 when I more or less discovered my "personal style", and there has been little deviation from it since. It's kind of Kurt Cobain-y (without the awesome obscure band shirts and patched jeans), in that I wear a pair of jeans, a pair of decent shoes, a plain t-shirt (usually non-graphic tee, though I am not "against" wearing one), and some sort of button-up/button-down (I still don't know the difference) shirt over the tee (be it a flannel, a mono-colored dress shirt, whatever - and usually unbuttoned). And that's really it. Not a very big fan of wearing a watch. I sometimes sport my "punker" hat that has shit drawn all over it, has studs stuck in it, and I am deliberately letting it get "old and crusty" for the sake of doing so. And sometimes during the summer, I will sport a tank top style shirt and jogging shorts. But aside from those hot/humid months in Summer, it is usually the stuff mentioned above.
So, I would say this is fairly "minimalist", as there tends to not be much deviation from this "style". And I am a cheapskate, too - I tend to wear just Fruit of the Loom t-shirts (I have three white v-neck tees arriving in the mail tomorrow, actually), and no one item in my wardrobe costs more than $45, probably.
But, it is good to make an effort to focus on one's "personal style" - what they gravitate towards naturally. What one likes, and feels comfortable in, and makes them feel like themself. And no one is "too old" (a foolish disposition to take to begin with) to have a "personal style". And style is wholesale different than "fashion" (which is superfluous, dumb, and sort of "thirsty", in my eyes). Taking care to take care is always of benefit to one's mental health, personal confidence, and just makes the individual feel better (and perhaps look better - though as long as one feels better, is all that matters) all around.