About the complex of inferiority "younger than me but surpasses me"
It's not only a pure incels' phenomenon. There are plenty of noncels, incels in denial, nearcels and others who had this complex (or at least short time feeling, thoughts) like "I am elder and I should be x (taller, stronger, more clever, etc.) but he/she surpasses me!" The difference is that some of the incels can describe it in other ways like "My younger cousin mogs me.", "My younger brother already mogs me.", "Even the teenagers around are mogging me.", "My students mog me already." and so on.
Why does this happen? The reason is the belief that "elder age = always better" which is only partly true. The elder age = more time to develop and of course more experience, hence in some cases it's a plus but not every time. For example if you spent your recent 5 years in playing video games and online complaining and meanwhile a younger person spent the same time (or even less time, for example -- 2 years only) to practice bodybuilding and a foreign language, then no wonder that he has a way better body and foreign language skills. Also there are some genetic traits which aren't very related to the age -- like the height, for example (yes, when he or she is 3 y. o. surely will be shorter than you but if his/her genetic height is "very tall" it will appear even before the puberty and when he/she is 10 y. o. you may see that the young person is already surpassing you especially if you're not very tall.)
So, as usual, all you should do is to try being a better version of yourself -- improve the things that you can improve (like your weight, muscles, knowledge and so on) and accept the things that you can't improve (like height, male pattern balding, etc.). It'll make you much better than someone who isn't improved (maxx-ed) and who on top of that has a complex of inferiority.
Do your best.