Expression: "He does not look a day older than 15 years."

[Tom]

Hi

Do you find the following sentence correct and natural?

[Conchita]

He doesn't look a day older than 15

sounds more natural to me.

(Sorry to be so nitpicking!)

[Tom]

[Yankee]

Hi Tom

In my opinion, your sentence would be most natural with the word 'over' (rather than 'older than'):

He doesn't look a day over 15.

No native speaker would ever think you were referring to 15 days or 15 months instead of 15 years. So, to any native speaker, the word 'years' is so utterly unnecessary that if you decide to add it, you will only succeed in making your sentence sound odd.

EDIT:
Also keep in mind that we basically never say "He is 15 years."
Instead, we say either "He is 15." -OR- "He is 15 years old."