Looking more mature is what you mean, looking "put together" is where you start. There are lot's simple tricks to do this effectively, and it starts with how you carry yourself.
If you consider the old fashioned thought of a 'finishing school', it is where young ladies were sent in the old days, and even now for a certain class, to go from girls to becoming mature adults ready to face the world.

Posture, how you walk, your vocabulary, your speaking voice, your laugh, your self control and how you present yourself; being calm & self assured.

These are all things that you have control over and can consciously work to improve.

The clothes you choose to wear, always appropriate to whatever situation, always modest and tailored to fit you perfectly. "You only have one chance to make a first impression" is the old saying, and it is a good one to tuck into the back of your mind to remind you when you shop or go out anywhere, that it is what you want to do. To make a good impression, always.

Hair and makeup are last, and again, the same holds true, appropriate, classic, well put together, tasteful, there are the things a 'mature' person would strive for.

Does that help at all, to help you focus on your goal to present your best self? Hope so. Best To You.