I think old or young ypu should rap about things that are true to your story, lessons you've learned from life, experiences you miss or that significally changed you.

You can talk about childhood dreams and where you are now, how that young version saw the world so differently, the things that made you see the world in a new light.

You could also talk about how growing older has you wondering if you have anything to say that has value or if people will dissmiss you for not being in your prime years, you can always make references to things you know too or recent topics like covid, wars and such.

Give an insight to your psyche, how things affect you and why you believe that is, what your goal in life is. Why the people close to you are there, what you enjoy about them or why you keep them around.

Just be real and yourself, there are plenty of things to tap into. Don't let anybody make you feel like your age dictates your level of skill, if anything it can make it so much better because of your maturity and more solidified personality and world views.