During my experience of consulting students , many times I came across parents and even students who are completely clueless regarding their UG courses/applications. Many of them even approach me after their 12th or A level to discuss on the college application, however they failed to understand that most of the applications start almost a year in advance and if you need to do standardised tests prep such as SAT and AP , it might take even longer.
So ideally it is advised that one should start exploring and gathering informed on his/her bachelor courses as early as when they are in secondary 4 or O level ( 9th-10th grade) to get the maximum leverage on their available time. Also if someone is looking for top University eg Ivy Leagues etc, they have to start paving the path quite in advance.
I believe college application is a parallel journey along with school curriculum and both are irreplaceable. Therefore right guidance and expert mentoring are the key for your success I. Getting your dream University.
Always remember no University is out of your reach unless you are well prepared for that!