Would it change the nuance of what Lemongrab's saying if I say "so young" instead of "too young" in this context?

Native speaker’s answer
Rebecca
Yes, it would change it a bit! "So young" would sound more like an observation, whereas "too young" means younger than the person should be, kind of like an objection. Ex: You're so young! You're younger than I thought you were. EX: You're too young to ride this rollercoaster. => one needs to be older