I'm of the mind that anyone under the legal age is a child. We know our brains don't finish growing until our mid-twenties, and anyone under that, I find myself not getting 'close' to. I'm a woman, though, and it's seen differently, I suppose, than a man befriending a teen. Still, I'd look at them like a little brother/sister and not as a friend, so to speak. When it comes to dating, as long as both parties are of legal age, it's not my business. Do as you please as long as both parties are consenting adults.
In Western countries, around 8% of male-female couples have an age difference of 10 years or more, reaching 25% in male-male unions and 15% in female-female relationships. For some, the gap is even greater: data suggests that around 1% of heterosexual couples in the US.