Example: I'm 21, but I'm not at that age yet where i can dress professionally (i.e. suits, etc).
I don’t think there’s an age limit (upper or lower) to dressing well, I think it’s more about when you decide to start putting some intention to what you put on in the morning.
Whether you’re still in school, or at a job that doesn’t require dressing up, it’s never too early to start migrating from the guys’ college uniform of ill-fitting jeans and a hoodie to a more grown-up wardrobe. Buy some nice shoes (you don’t need to jump right to a wingtip - start with a desert or chukka boot) and put away your sneakers except for on the weekends. Throw on a lightweight blazer or v-neck sweater when layering, as opposed to the aforementioned hoodie or track jacket. Depending on what kind of music you’re into, size down from wide leg jeans into a slimmer cut, or, conversely, go from a skinny style to one slightly less so.
Most importantly, it’s great that at 21, you’re thinking about how your clothes reflect on you in the professional world or otherwise. It takes a lot of people a lot longer than that to figure out that dressing well helps you (1) project the confidence of someone who deserves the corner office and (2) lets others know you’re not just there for a spring internship. If you start to incorporate “grown-up” clothes into your wardrobe now, it won’t feel like such a shock when you do get out of school or graduate to a more buttoned-up office and can’t roll out of bed and into sweats every day.