Baby-face

Because he has a baby-face, Tom is always asked to provide ID when he goes out to buy alcohol.

Source: tatoeba (10151952)

[T]he jolly, broad, fooliſh, humorous, half-laughing, half-crying, baby-face of the Major extorted peals of laughter from all who were preſent.

Source: wiktionary

Say that the Cherubs carved in stone, / [...] / Used to sing in heavenly tone, / Above and round the sacred places / They guard, with wingèd baby-faces.

Source: wiktionary

Now Sir Wigolais was not only very young, but extremely young-looking; he had one of those baby-faces that obstinately refused to look manly at any age, and a chin that seemed destined never to wear a beard.

Source: wiktionary