@ CALL FOR CHANGE
first of all welcome to you on this prestigious forum of learning and knowledge where u will find lots of helpful members who will try sincerely to remove all of ur confusions.
secondly, dont be confused. calm down ur nerves and start by visiting the beginners guide on the website and then visit the syllabus link and then think that which optional subjects are most suitable to ur interest and then visit all the psat past papers of previous years for both compulsory and optional subjects and then start ur preparation in a smooth n strategic manner. try to make ur own notes.

as u mentioned that u have no friends who r intersted in CSS so no problem . plz accept my offer of freindship or otherwise u will b able to make many other friends on this forum.
as far as ur question about age limit is concerned , yes of course there is restriction regarding age limit which is from 21 years to the age of 28 years. in this limited time span of 7 years u can make three attempts means u have three chances to appear in the exam . although there is age relaxation but in some special cases.

best of luck for ur CSS.
ALLAH HAFIZ.