pero tu DEBES decidir a con clase
i would change
Comprension esa tu can desear joven, per sagaz y teatro tu real anos
En mi opinion, tu solo eres tan joven que tu sientes.
Also, in some instances you're using the TU form and in others you're using the USTED form so i would edit it to make them either alll TU or all USTED to make the whole paragraph aggree
En general hay nada incorrecto con UN DESEA PARA MIRAR joven.
hay nada incorrecto con desear mira = don't forget the double negative in Spanish = no hay nada incorrecto en desear parecer joven.
Brianna is absolutely correct in that must decide first of all if you are writing this with the familiar tú form or the formal usted form. Next, do NOT write it out in English first and then try to translate word for word (literally) because that never works. SIMPLIFY what you are trying to say and use the structures and vocabulary you have mastered in class. There is so much wrong, and in some instances I don't understand your English , that I'm not quite sure how to help you in some spots.
Pero tú debes decidir hacerlo con clase, comprendiendo tu edad.
No hay ninguna razón de hacerte daño, sólo para sentirte joven.
Pienso que sólo eres tan joven como te sientes.
Tratando de parecer más joven puede también lastimarte.
Nadie quiere pasar tiempo con alguien que se porta como si tuviera dieciséis años.
When you say you don't understand the verb + subject thing, here's an example: Nadie (subject) desea (verb with personal ending to agree with that subject.
Now, it will depend upon what level of Spanish you are in. The "como si" construction does not appear until the end of Spanish III. If you are not in that level, you'll have to reword it to avoid that pitfall. (as if...)
Feel free to ask any other questions, but please don't forget the advice to SIMPLIFY what you are saying! Just accept the fact that you may never say exactly what you'd like in your native language.
P.S. Feel free to change anything you'd like and repost the final paragraph for a final proofreading.