If you think about it, our souls don't age, only our bodies do. Our souls/spirits accumulate "time in existence" but they never get "old" because they'll exist forever
Living in a sinful, fallen world is what makes anyone feel old, whether it's through health issues or mental/emotional fatigue, etc. I know 20 something's with very little will to live, and I know 90 something's with more pep that I've ever had, lol.
I think the key to a youthful spirit is refusing to "get old", if that makes sense. We often equate aging with becoming crotchety, cranky, stubborn, set in our ways, cynical, etc. Making a conscious effort to remain joyful, light-hearted, playful, curious, and open-minded is what keeps me feeling like a kid. Of course, we can't stop our bodies from aging, but we CAN control our attitudes and thoughts, our choices, and our outlook on life.
I also refuse to "become a woman" in some ways
I don't wear much makeup, I don't do anything with my hair, I don't own a dress or a pair of high heels, my favorite summer outfit is a tank top, shorts and bare feet, and I have zero desire to ever learn about scrapbooking, baking, or fashion. I've retained my tomboyish nature all my life, and I hope I never lose it
There's something about being a tomboy that smacks of youthfulness (at least to me). The day I put on a dress and heels and start baking cookies.. I will start getting old lol.
I want to be the old lady in jeans and a cute tshirt with undyed gray hair, a snappy joke, an easy laugh, and an endless appetite for adventure and new experiences.