Of course, this is all subjective, but I would say that the ideal time to start thinking about college is in 8th grade. College admissions officers look at everything you’ve done in high school (9th - 12th grades), so right before high school is a good time to start taking your activities and academics more seriously. It’s always good to enter high school with the target of getting the highest possible GPA. If you relax the first two years and then only get serious in 11th and 12th grades, your transcript may be damaged by grades in classes you regret not studying harder for. Summer programs are also something to start considering in 8th grade (or earlier, if you’re especially ambitious). Summer programs are like little “treats” that you can add to your resume to demonstrate your enthusiasm for academics and strong work ethic. In addition, building extracurriculars is something you should consider starting early. If you have too many “scattered” activities on your resume that you hastily joined in high school but never got especially involved in or especially good at, this weakens your profile on college applications. It is better to excel at an activity (for example, science fairs, geography bees, tech competitions, sports, publishing research or writing, etc) than to be mediocre at eight different activities. If you’re not all that enthusiastic about attending a good college, then these are probably not things you need to worry about.
I realize that my personal example differs greatly from others’ examples, but I’ll write about it here just to paint a general idea of what college preparation looks like years in advance. I started fencing when I was eight and started competing in national and international competitions early. I trained 5-6 days a week from 7pm-10:30pm and took 3 private lessons a week with my coach outside regular training hours. I never was home during Thanksgiving because of the annual World Cup in Germany I went to, and I was never home during Christmas break because I traveled to different fencing clubs in other states to attend their winter camps and practice with competitors I didn’t normally see at my own club. This was the routine I followed as soon as I reached 8th grade. In school, I maintained great grades and took studying seriously. I also attended summer academic programs at Choate Rosemary Hall. On my own time, I was involved in Girl Scouts and volunteered at local charities.
Not everyone has to follow this template, but I believe that regardless of your interests and activities, it is never a bad idea to start thinking about college when you reach 8th grade. If you have an idea of what you want to do in 8th grade, then planning your high school track will be much easier.