Idd say that Link is 10 as a child and 17 as an "adult"
If it is called "adult link" then that means he is over 18. Unless Hyrule has a different age for becoming an adult. I doubt that Link is 20 or older as adult Link. So young link is probably about 11 or 12. Based on the voice acting, he sounds exactly like my younger brother's friend. Who is 12.
Link's age in Ocarina of Time has actually been confirmed by Miyamoto in a recent interview with Satoru Iwata. This is what Miyamoto said in the interview: "When we decided to handle Link growing up from a 9-year-old child to a more mature 16-year-old". The interview also mentions Link's age in the original Zelda game and in Zelda 2. Here is the link to the interview: Iwata Asks.