Imagine someone whose voice is too young for their age; which one of the following self-made sentences below sound more natural to you in such a case:
- You sound a lot younger than your age.
- Your voice is too young for your age.
You have a young(-sounding) voice.
You sound(ed) very young on the phone.
Of course, you could say something like:
Wow, you're a lot older than I expected after hearing you on the phone!
but it can be impolite to imply someone is old. Wherever possible (in polite conversation) you want to emphasize the other person's youth over their maturity. It is difficult to do this with someone's voice however, since there's an implied contrast between how they sound and how they look.
Personally I'd avoid saying someone's voice is "young" and instead focus on its tonal qualities, or a general comment about the difference in expectation:
Your voice is a lot higher than I expected.
You don't look how I thought you would look.
Given this is a potentially uncomfortable topic, I might avoid it completely, though.
- Oh, you sound so young!
- You sound (much) younger on the phone
- He had a youthful voice.
From Long, Lean and Lethal
By Lorie O'Clare
Betty, the dispatcher, had a natural high pitch to her voice, making her sound younger on the phone than she actually was.
Sentence 3 is perhaps the least common, but I like it.
The first sentence is correct grammatically, but the verb look is more common and idiomatic in the sentence as follows:
You look much younger than your age/years.
The second sentence may be rephrased as follows:
Your voice makes you sound much younger than your age.