Speaking from experience, being the youngest appears to have no upsides: you're expected to perform as well as people older than you which creates tons of pressure, you're probably physically smaller which can impact confidence/sports/romance, socially you're a year behind, and it only gets worse later - in many countries the legal drinking age (18) aligns with the average age of entering university which means your options for social development at a crucial stage of development are restricted further, etc.
By the time you're 30, an extra year saved going to school earlier means nothing, and the price you paid in terms of developing neurotic conditions (e.g. perfectionism) which impact confidence and social skills, is most definitely not worth it. My kids will follow a similar path to the one you outlined, and I will strongly encourage them to take a gap year before considering applying to university.
You're projecting. My experiences as the youngest, and smallest in the class until the last year of high school, left me with the following experience - No impact, no differences based on being younger and smaller.
I would say what you're saying are results of being younger are actually results of your personality traits, and being a year older would have still lead you to having similar experiences at school.
By the time you're 30, an extra year saved going to school earlier means nothing, and the price you paid in terms of developing neurotic conditions (e.g. perfectionism) which impact confidence and social skills, is most definitely not worth it. My kids will follow a similar path to the one you outlined, and I will strongly encourage them to take a gap year before considering applying to university.