Como dizer "saiu das fraldas" e "bem crescidinho" em inglês

Para dizer "acabar de sair das fraldas", "ainda usa fralda", etc., no sentido de ser ainda muito jovem e sem muita experiência, sugiro "wet behind the ears".

Em relação a "bem crescidinho", com o sentido de "já adulto", "maduro" e ''pronto'' para fazer alguma coisa, sugiro o uso de "quite grown-up". No entanto, no contexto da pergunta, "too" no lugar de "quite" torna a frase mais natural.

Wet behind the ears: the allusion is to the inexperience of a baby, so recently born as to be still wet. [phrases.org.uk]; young and inexperienced. [Thefreedictionary]

Grown-up: like an adult. [Merriam-Webster]

Exemplos de uso:

You're quite grown-up.
Você está bem crescidinho.

You're too grown-up to get an allowance.
Você já é bem crescidinho para ganhar mesada.

He's still wet behind the ears but wants to drive.
Ele acabou de sair das fraldas e já quer dirigir.

Exemplos (referências) adicionais:

1. The job put a lot of responsibility on someone who was still wet behind the ears, but he learned fast. [Thefreedictionary]
2. How can you take instructions from Tom? He's still wet behind the ears. [The American Heritage® Dictionary of Idioms by Christine Ammer.]

Bons estudos.

He's still wet behind the ears but wants to drive. = Surprisingly, this is a cowboy saying. It refers to a calf so young that it "...is still wet behind the ears."

He Is still in kneepants. = I assume this expression is from the 1800s when boys wore pants that ended at about the knee.