Usually, with brighter colors, it becomes more difficult to determine ages by just looking at characters.. For example is many Disney characters. Many of the girls are only around 16, but they look old enough to be legal. >_> With some Disney males, some look younger than their age (Aladdin looks 16 when he is 18) while others appear to be older than their age (Beast looks 23-25 as a human but he's 21). Because of bright colors and styles that lean to animated looks rather than realism, we get the "exaggeration" that the creators have been talking about. So, with exaggeration of styles comes exaggeration with characters' physical features. This is why it's a little hard to tell how old SS Link might be just by looking.
Also, the in-game art never matches up with the hand-drawn art. Remember that cool picture for TP with half-Link, half-wolf-Link? That Link looks in his twenties in hand-drawn art, but he's only 16! With SS, the hand-drawn art he looks.... 23,24? But he is probably younger. I don't think they would make him mucher older than 18. Any older, and he becomes too much of a man. It's always a story about a boy, remember? He might be a young child or an older teen, but he's always somewhat still a boy. Making him in his twenties would probably make him lose that innocence that's always left in tact. Maybe that might work for another T-rated game in the future, but I don't think it'd work well this this art style.
I really don't think he'll be in his twenties, probably just 18 or 19.